Well, it depends on what you want. You can add all kind of customisation to any website.
E.g. just recently YouTube changed its theater mode (which I use to have a larger video in non-fullscreen mode):
They’ve removed margins on the sides of the page making everything going to the edge of the browser window. But it doesn’t improve anything for typical 16:9 videos, in contrary it’s very irritating for me personally because I use the (right) side for scrolling with my thumb (tablet) and then I often accidentally hit video recommendations and navigate away…
Aaanyway: I previously didn’t mention that if you just want to override CSS this can be easier accomplished with user styles.
I use an extension called Stylus for managing such styles (FireFox/Chrome).
I’ve put the styles I use for this particular forum on userstyles.org:
Thimbleweed Park™ Forums fixes
Note: Click on Advanced Style Settings to let the fun begin!