Hey cool, I just played around with it too
The bold formatting works perfectly!
It only seems the background doesn't work on ALL pages, e.g. not in tables, so I replaced the "body" with "*"
* { background-color: #FAE0C5 !important; font-weight: bold !important; }
For the font type and size there are also some settings here:
Edit / Preferences / Page Display / Fonts
There can be set a minimum size for the fonts, but somehow it doesn't always seem to work or probably it's me. Never mind, there's another way:
Type "about:config" in address bar (or Edit/Config./Browser Config.)
Type "minimum" in the second line and wait a second, then doubleclick on the two entries and change the size, e.g. to 14
Unlike the style sheet this setting works instantly without a restart.
Oh man, but now I gotta switch back to normal display, 'cause all this bold and the uniform tables without any different colors in their lines somehow are stressing MY eyes, LOL!