(The in-game config in the Generic Mod Config Menu includes detailed descriptions that should be self-explanatory, but if for some reason you want to edit the config.json file directly instead of using GMCM, this explanation is here for you)
The config is long and rich, and divided into several sections: the unnamed main settings section at the top, then ***Pajama controls, ***Outfit names, ***Festival outfits, and finaly ***Seasonal variations.

!Note that the config is global for all saves. If you're loading a different save and the mod doesn't seem to be working, go through the config and make sure the settings are correct and the outfit names match the currently loaded save.

*Use mod outfits
Turns on the Fashion Sense half of the mod. On by default. Note that you can toggle this at any time, as long as the required framework mods (Fashion Sense and FashionSenseOutfits) are installed. Possible values are "true" and "false". Enabled by default.

*Use mod portraits
Same as above, the required framework mod being "Farmer Portraits".

*Seasonal outfits
The farmer will attempt to wear different oufits and load different portraits depending on the season. If disabled, a "base" portrait and outfit will be shown, portrait_png and "base" by default. To customize Fashion Sense outfit names for different seasons (for example, to use the same outfit in three seasons and a different one in winter), scroll down to the corresponding config options. Default is "spring", "summer", "fall", "winter". These are also the default portrait file name postfixes. This outfit will be overwritten by any other option below that's enabled: raincoat, pajamas, etc. Possible values are "true" and "false". Enabled by default.

*Raincoat enabled
Turns on the "raincoat" feature: a special outfit (the default portrait file name postfix and outfit name are "rain", can be customized further down the config) will be worn when it's raining or storming in Stardew Valley. Overwritten by any other enabled option below this one: pajamas, a festival, etc. Possible values are "true" and "false". Enabled by default.

*Seasonal raincoats
Switches from using a single raincoat for any season to season-specific raincoats. Disabled by default - by default, a single raincoat is used. Possible values are "true" and "false". Doesn't do anything if the raincoat option above is disabled. Default outfit names are "SpringRain", "SummerRain", "FallRain", "WinterRain", default file postfixes are "rain_spring", "rain_summer", "rain_fall", "rain_winter", outfit names can be customized further down in the config. Possible values are "true" and "false". Enabled by default.

*Festival outfits
Turns on the "festival outfits" feature: you can set up a single festive outfit to be worn when anything at all (that the game registers as DayEvent) is going on, and you can set up outfits for individual festivals. Default file postfix for each is its DayEvent value in the game, which for vanilla festivals is their full name, e.g. "dance of the moonlight jellies", "spirit's eve", etc. See default outfit names further below, where they can be customized. Custom festivals can be added in the code manually, in the content.json file: open it and search for "//custom festival code sample", then follow the instructions there. Possible values are "true" and "false". This outfit will be overwritten by any other option below that's enabled: mining, desert, pajamas, etc (so you can visit Sandy on an Ice Festival morning without swearing in your winter coat). Enabled by default.

*Mining outfit enabled
Turns on the "mining outfit" feature: a special outfit (the default portrait file name postfix and outfit name are "mining", can be customized further down the config) will be worn in the "dungeon" locations: the mine, the quarry cave, the Skull Cavern and the volcano. Custom dungeon locations can be added in the code manually, in the content.json file: open it and search for "//add custom mining locations". Downtown Zuzu, East Scarp and Cape Stardew locations should already be included, although mistakes are possible. Tell me in the comments on the mod page if something is wrong! (https://www.nexusmods.com/stardewvalley/mods/19503?tab=posts) Possible values are "true" and "false". Enabled by default.

*Mountain mining
Has the farmer wear the mining outfit also to the "mountain" location - the map with the quarry, the Adventurer's Guild and mine entrance, the mountain lake, Robin's house and Linus's tent. Does nothing if the mining outfit is disabled. Possible values are "true" and "false". Disabled by default.

*Seasonal mining outfit
Has the mining outfit vary by season. See the seasonal variations section below.

*Desert outfit enabled
Turns on the "desert outfit" feature: a special outfit (the default portrait file name postfix and outfit name are "desert", can be customized further down the config) will be worn in the Calico desert, including Sandy's shop and the casino. Custom desert locations can be added in the code manually, in the content.json file: open it and search for "//add custom desert locations". Downtown Zuzu, Walk to Desert Redux and Desert Afforestation Area locations should already be included, although mistakes are possible. Tell me in the comments on the mod page if something is wrong! (https://www.nexusmods.com/stardewvalley/mods/19503?tab=posts) Possible values are "true" and "false". Enabled by default.

*Island outfit enabled
Turns on the "island outfit" feature: a special outfit (the default portrait file name postfix and outfit name are "island", can be customized further down the config) will be worn on all Ginger Island maps (including custom ones, as long as the LocationContext is correct).

*Island raincoat option
Selects which raincoat you wear on the Ginger Island when it rains: you can hook it up with the regular raincoat or the seasonal raincoat, you can set its own raincoat (default outfit name "IslandRain" and default portrait file name postfix island_rain), or you can turn it off and run around in your regular island outfit (or your regular outfit period). Possible values are "None", "Regular raincoat", "Seasonal raincoat" and "Island raincoat". Disabled by default (the "None" option). Always does nothing if "Raincoat enabled" is deselected.


***Pajama controls config section
        
*Morning pajamas
Turns on the morning half of the "pajamas" feature: a special outfit (the default portrait file name postfix and outfit name are "pajamas", can be customized further down the config and for the portrait file name, after turning off the "legacy portrait file names" option) will be worn in the farmhouse and the island farmhouse in the morning upon waking up. Possible values are "true" and "false". Enabled by default.

!Note that due to some inconsistent behavior of the FashionSenseOutfits framework it's recommended to use both the morning pajamas and the evening pajamas feature: when loading a save, you will wake up in the same outfit you went to bed in, even when it would normally be switched on starting a new day if you were playing continuously, and it will not switch to what it should be until either time ticks over (6:10am) or you switch locations (leaving the farmhouse or going to the cellar). Going to bed and waking up in the same pajamas makes the issue fully invisible.

*Pajamas until
Sets when you change out of pajamas and into your outfit for the day when inside the farmhouse (leaving the farmhouse changes you out of pajamas regardless, but if you go back in and it's still pajamas time, you'll be wearing them again). It's a multi-select instead of free text entry because I have to correspond each option to in-game time manually, because json is not a real programming language. You understand. Possible values are "6:10", "6:20", "6:30", "7:00", "7:30", "8:00", "8:30", "9:00", "10:00", "11:00", "12:00". Default is "6:10". It's all in the AM, obviously, because it's about wearing pajamas in the morning.

*Evening pajamas
Turns on the evening half of the "pajamas" feature, see above. Has you wear pajamas in the farmhouse before you go to bed; together with morning pajamas constitutes an effective workaround to the FashionSenseOutfits save load issue. Possible values are "true" and "false". Enabled by default.

*Pajamas from
Sets when you change from your outfit for the day into pajamas when inside the farmhouse in the evening, same as "pajamas until" above in all respects, except time is first in PM and then in AM as we cross the midnight point. Possible values are "9:00", "9:30", "10:00", "10:30", "11:00", "11:30", "12:00", "12:30", "1:00", "1:30", "1:40", "1:50". Default is "10:00" (10 PM, obviously).

Note that the pajamas feature overrides all other outfit options: as long as you're in the farmhouse or the island farmhouse and it's the right time, you will be wearing pajamas regardless of the season, festival, whether you added your farmhouse as a mining or desert location for some reason, etc.

***Outfit names config section

!Note that the config is global for all saves. If you intend on switching between different saves with different wardrobes, you'll need to either use the same outfit names in all of them (such as the default ones), or change the settings every time you switch.

*Base outfit
For the outfit you wear day to day when you aren't wearing anything else, sets the name of the Fashion Sense outfit in your wardrobe as well as the portrait file name postfix. Only actually relevant when you're not using the seasonal outfits feature, since there's always SOME season and that's what you'll be wearing instead. Use example: entering "overalls" into this setting will have the mod load the "overalls", "Overalls", "OVERALLS" or however else you capitalize the outfit name from your Fashion Sense wardrobe, and "portrait_overalls.png" (or "pOrTrAiT_oVeRaLlS.png" or however else you capitalize it) from your portraits folder. Note that it will load "portrait.png" first in all cases and missing the "portrait_overalls.png" file will not cause an error, the previous portrait (directly portrait.png in this case) will be loaded instead. The default value is "base".

*Pajamas
For the outfit you wear in the morning and/or evening, if enabled, sets the name of the Fashion Sense outfit in your wardrobe as well as the portrait file name postfix. See *Base outfit for use example. The default value is "pajamas".

*Raincoat
For the outfit you wear when it's raining, if it's enabled and the seasonal raincoats option is disabled, sets the name of the Fashion Sense outfit in your wardrobe as well as the portrait file name postfix. See *Base outfit for use example. If the "island raincoat option" is set to "regular raincoat", it will use this outfit name even if you're wearing seasonal raincoats on the mainland. The default value is "rain".

*Mining outfit
For the outfit you wear while mining/dungeoneering, if enabled, sets the name of the Fashion Sense outfit in your wardrobe as well as the portrait file name postfix. See *Base outfit for use example. The default value is "mining".

*Desert outfit
For the outfit you wear in the Calico desert (and other desert locations if you have any in your game, see *Desert outfit enabled above for instructions on adding custom locations to the list), if enabled, sets the name of the Fashion Sense outfit in your wardrobe as well as the portrait file name postfix. See *Base outfit for use example. The default value is "desert".

*Island outfit
For the outfit you wear on Ginger Island, if enabled, sets the name of the Fashion Sense outfit in your wardrobe as well as the portrait file name postfix. See *Base outfit for use example. The default value is "island".

*Island raincoat
If you have selected the "Island raincoat" option in the corresponding setting, sets the name of the Fashion Sense outfit in your wardrobe as well as the portrait file name postfix, for the raincoat you wear when you're on the island and it's raining there. See *Base outfit for use example. The default value is "islandRain".
        
***Festival outfits

!Note that this entire section doesn't do anything if the "Festival outfits" option is disabled in the main config.

Note that all festival outfits are overwritten by more specific outfits: island, desert, mining, pajamas, etc. They will however overwrite the raincoat, if your modded game makes it possible for it to rain during a festival (not possible in the vanilla game), and of course they overwrite seasonal outfits.

*Single festive outfit
If this option is enabled, you will wear a "default" festive outfit for all occasions that have a DayEvent value in the game (vanilla festivals, weddings, anything added by mods). Disable it to default to your regular outfit on occasions you aren't interested in and won't set the value for below. Note that adding specific custom festivals to this mod's code is relatively easy: open the "content.json" file in the mod folder, search for "//custom festival code sample", then follow the instructions there. Of course, using this option is undoubtedly easier. Possible values are "true" and "false". Enabled by default.

*Festive outfit
For the outfit you'll wear on festive occasions by default, if enabled above, sets the name of the Fashion Sense outfit in your wardrobe, and if the "legacy file names" setting is off, also the portrait file name postfix. See *Base outfit for use example. The default value is "festive".
        
*Wedding outfit enabled
When enabled, makes you wear a special outfit on your wedding day. (Note that this is for the whole day, not just the ceremony; I don't think Content Patcher allows me to separate it out). If it's disabled, you will either wear the "default" festive outfit on your wedding day (if the corresponding option is enabled) or your regular seasonal / default outfit (if it isn't). Possible values are "true" and "false". Enabled by default.

*Wedding outfit
For the outfit you wear on the day of your wedding, if enabled above, sets the name of the Fashion Sense outfit in your wardrobe as well as the portrait file name postfix. See *Base outfit for use example. The default value is "wedding".
        
*Egg Festival outfit enabled
When enabled, makes you wear a special outfit on the day of the Egg Festival. If it's disabled, you will either wear the "default" festive outfit on that day (if the corresponding option is enabled) or your regular seasonal / default outfit (if it isn't). Possible values are "true" and "false". Enabled by default.

*Egg Festival outfit
For the outfit you wear on the day of the Egg Festival, if enabled above, sets the name of the Fashion Sense outfit in your wardrobe as well as the portrait file name postfix. See *Base outfit for use example. The default value is "EggFestival".
        
*Flower Dance outfit enabled
*Flower Dance outfit
Same as above, default is "FlowerDance".

*Luau outfit enabled
*Luau outfit
Same as above, default is "Luau".

*Dance of the Moonlight Jellies outfit enabled
*Dance of the Moonlight Jellies outfit
Same as above, default is "MoonlightJellies".

*Stardew Valley Fair outfit enabled
*Stardew Valley Fair outfit
Same as above, default is "Fair".
        
*Spirit's Eve outfit enabled
*Spirit's Eve outfit
Same as above, default is "SpiritsEve".

*Ice Festival outfit enabled
*Ice Festival outfit
Same as above, default is "IceFestival".

*Feast of the Winter Star outfit enabled
*Feast of the Winter Star outfit
Same as above, default is "WinterStar".

!Note: this is the part of the config where the custom festival setup will go if you add any. To do so, open the "content.json" file in the mod folder, search for "//custom festival code sample" and follow the instructions there.

***Seasonal variations config section

*Spring outfit
*Summer outfit
*Fall outfit
*Winter outfit
For the outfit you wear in the respective season when nothing else that overrides it is going on, sets the name of the Fashion Sense outfit in your wardrobe as well as the portrait file name postfix. See *Base outfit for use example. The default values are "Spring", "Summer", "Fall" and "Winter" respectively. Doesn't do anything if the "Seasonal outfits" option is disabled in the main settings section.

*Spring raincoat
*Summer raincoat
*Fall raincoat
*Winter raincoat
For the outfit you wear in the respective season when it rains, sets the name of the Fashion Sense outfit in your wardrobe as well as the portrait file name postfix. See *Base outfit for use example. The default values are "SpringRain", "SummerRain", "FallRain" and "WinterRain" respectively. Doesn't do anything if the main "raincoat enabled" setting is off, and also if you don't have seasonal raincoats enabled in the main settings section or chosen as an island raincoat option. (I have no idea why you would use seasonal raincoats for the island but a single raincoat for the mainland, but you could in fact do that)

*Spring mining outfit
*Summer mining outfit
*Fall mining outfit
*Winter mining outfit
For the outfit you wear while mining in the respective season, sets the name of the Fashion Sense outfit in your wardrobe as well as the portrait file name postfix. The default values are "SpringMining", "SummerMining", "FallMining" and "WinterMining" respectively. Doesn't do anything if the mining outfit is disabled, as well as if the seasonal mining outfits option is disabled.