Good stuff.

I'll get to the line height and footer soon.

I recommend disabling that "Moderator Of" feature until too many subs becomes a problem, a problem only a few might have. It might be better to have columns or treat it like the wall of text box below the submit new post with all "your subs". FYI, in the CSS I made that box width:100% and it word-wraps fine (without sidebox overlap issues) so it makes better use of the page.

I hadn't intended for a custom site-wide CSS thing but that sounds great too.

I had intended to mean a custom CSS that applied to your personal user pages only.

I don't know which is easier, or if one is easier, or what, but one or both would be neat options in preferences.

One user-wiki is good. More user-wikis might be better, or the option to make several. With CSS even better. And to add images or media even better, though you may wish to limit their gallery, depending on your resources.

For what it's worth, I already have a user wiki so I wouldn't use it beyond the basics, though others may, and it might be appealing to those wanting more in their social media. I don't need user-custom CSS either but I would apply it. It'd be nice to have with a corner switch like night mode.

5000px wide. Wow. I thought I was overdoing it with 2200px wide banners. All my browsers are at 90% for more and smaller text so I extended a little in case anyone else might too, though I rarely fill a wide monitor with a browser.

Can CSS scale images? There are several images where this might be useful. Granted too much scaling up and the bits would show. (Vectors rule.)

Can text be auto-scaled? For example, I might like to use an empty speech bubble and put the vote numbers over it. The greater the number the smaller it'd have to be to fit.