At line 4183 (in the version I have not too old) it seems like a tag isn't closed that should be so one media query is running into the next.
}
.topic_name, .topic_starter {
margin-right: 18px;
}
/* This one does 540 and smaller screens at default font size. */
@media screen and (max-width: 33.750em) {
It seems to me the .topic_name, .topic_start clause isn't closed correctly, so the media query above it isn't closed correctly.
ETA: As long as I'm here, in the max-width 33.750em media query you define the font-size of .topic_name h4 as 1.357em on ~line 4279 and then a bit later as 1.786em on ~ line 4379. Personally I like the 1.357 but whichever way you go probably only need to set it once.