Well, got my answer... There was a JavaScript error, which occurred on all mobile themes, if the user wasn't logged in, and thus had no rights. (It wouldn't happen on boards where guests are allowed to post, but it doesn't matter.)
And as I expected, fixing this bug suddenly makes the CSS fail in guest mode as well. Meaning, well... That it's all due to a JavaScript problem in the first place, and CSS hacks won't help at all, eh.
Was looking in the wrong place... Now I need to determine what JS function changes anything in that area. My bet is on mini-menus, of course.
:edit: Confirmed. Disabling mini-menu code will make it work. Uh. So, it's back to being a CSS problem, I guess... :^^;: (As: CSS added by the JS function.)