uhm. Nevermind.
For anyone as stupid as me, here’s the solution:
Apparently, for a looong while I successfully forgot about and ignored the first line in my custom stylesheet:
@media all and (min-width: 1201px) {
#main {
width: 60%;
margin: 0 auto;
}
So of course, all my changes were applied not global, but only to 1201px devices and more. I put the line at the end of the custom css file, and now everything works like a charm.
Sorry for bothering you!