This task is now solely about this issue:
Special:Preferences layout in Monobook is hard to use with OOUI (and its Monobook theme Apex)
There is way too much whitespace, and it is wrapping the body of the page too narrow compared to the rest of the page. Two examples given.
The button layouts are also confusing, not being oddly located to the side of large text blocks, and the help is way in the bottom corner.