What we call "System" theme should have very minimal edits over the
actual system theme. So let's drop all the "font-size" properties (one
"larger" one, but especially the many "smaller" ones).
This means that the "Default" theme also will keep system font size.
This is as discussed with Ville and Liam, the later saying he can barely
read dockable texts because of this.
On the other hand, we use "small" font size as a general rule in the
"Compact" theme, which is especially meant for people who want a compact
theme.
Something which is regularly asked is to use smaller icons. We used to
have a "Small" theme in 2.8, which got away in 2.10 with icon sizing (cf
issue #6121).
This is a revival of such a theme (except I call it "Compact" because it
feels more appropriate than "Small"). The theme is basically the same as
"Default" (through an include), just tweaking some size related CSS
rules.
