An area in either Settings or Edit Profile to put CSS in that will be applied site-wide whenever you're logged in.
I'm not a big fan of dropshadows and larger rounded corner radii, and the blurring that gets applied when I click the "new post" button in the bottom right corner causes some performance issues on some of my machines. I'd like to change that without making requests that other users may disagree with, and I'd like the changes to be present on systems where I don't have access to browser userstyles.
I just know someone's going to make a userstyle that turns cohost into Windows XP and that's what I really want. I could also imagine a community-run theme gallery.
I use the search engine "Kagi", which introduced this feature a few months back, and I absolutely love having my own CSS customisations present despite not having a browser extension to enable userstyles.
here to second the hope for a community-run theme gallery someday