Okay, let's try this from the top:
1) All CSS served to the browser viewable via the source code. Yotsuba B:http://static.4chan.org/css/yotsubluenew.489.css
2) MLPchan thread pages have different markup than 4chan pages. You shouldn't expect to be able to copy the stylesheet verbatim and get an identical look.
3) The code you're looking for is:
Use of !important is discouraged because it may interfere with other settings end-users have their browser (Nielsen). Just put the modified stylesheet at the end of the stylesheet list in the page head. If two selectors have the same level of specificity, the last selector takes precedence.