Bad news, the game's shaded UI backgrounds are screwed up if you ain't on 16:9/16:10.
Good news, we can fix that by editing a single mesh file!
For those curious, here's how it works: the HUDGlassFlat.nif file has four vertices in it. The vertices' X positions are what need to be changed. By default, they're set to a multiple of 16/9 (1.7778); 132.1250.
We take that number, divide it by 1.7778, and we get 74.3193. This is the base value for the X axis. We then multiply that by 5:4 (5/4 = 1.25), which gives us 92.8991. Punt that in, save nifskope, et voila. Fixed menu backgrounds.