You probably know that you can tweak appearance of websites with add-ons like Stylus. Ever since I figured out how to use Inspector and how to write "installable" CSS stylesheets myself, I cannot resist the urge to fix websites with a uneasy appearance. I try to fix it with a few lines of CSS and share it with others.
GitHub quickly rolled out a new wider interface, which could be done a bit better, IMHO.
This is more advanced as it uses configurable parameters. The CSS rules are also less hacky, thanks to identifiable CSS classes in Mastodon's frontend.
The stylesheets are CC-BY-SA licensed. Hope it is useful for you.