userContent.css example
Changing the site without any addons in Firefox (guaranteed to work on Firefox 61.0.2 but should work on other versions, too):
In Linux create a file ~/.mozilla/firefox/<your-profile-id>/chrome/userContent.css
(I do not know the correct folder on Windows). In this file you can specify rules for specific domains with
@-moz-document domain(tex.stackexchange.com), domain(tex.meta.stackexchange.com) {
/* your rules here */
}
On Windows you can find the correct folder to put the userContent.css
into with https://superuser.com/a/319322/393639 (thanks @BMWurm for sharing that link).
In it you can specify any css content (I don't think that functions work). Changes to that file require a Firefox restart to take effect.
E.g. my current file looks like this:
@-moz-document domain(tex.stackexchange.com), domain(tex.meta.stackexchange.com) {
.post-tag::before { content:"{"; }
.post-tag::after { content:"}"; }
.post-tag {
border-width:0pt !important;
color:#444 !important;
background-color:transparent !important;
padding:0pt !important;
}
#sidebar {
max-width:20% !important;
}
.module.community-bulletin{
background-color:#fdfbed !important;
}
#sidebar .related a.question-hyperlink{font-size:12px !important;}
#mainbar { min-width:77.5% !important; }
.question-hyperlink {
font-size:16px !important;
font-family:'DejaVu Serif' !important;
}
.s-btn {
border-style:none !important;
border-radius:0px !important;
}
.kwd, .dec {
color:#894a16 !important;
}
.pun {
color:#c11c21 !important;
}
.prettyprint {
line-height:1.5 !important;
}
.vote-accepted-on, .vote-accepted-off, .vote-up-on, .vote-up-off, .vote-down-on, .vote-down-off, .star-on, .star-off {
background-image:url("http://cdn.sstatic.net/Sites/tex/img/sprites.svg") !important;
}
.vote-up-on, .vote-up-off, .vote-down-on, .vote-down-off {
height:48px !important;
}
}
Results with my file:

Credits for all those changes go to the other answers in this thread, I just tweaked some of them. Also @William provided
#sidebar .related a.question-hyperlink{font-size:12px !important;}
Which lets you choose a different font for the sidebar's community bulletin.