Starting a thread for people to post CSS tweaks for darktable. Let’s try to keep them small and isolated if possible (don’t post everything at once) and if you could post a before/after screenshot that would be even better!
A couple to start off…
The following CSS changes the font for the input box in which you edit your CSS tweaks (a tweak for the tweaks):
#usercss-box textview { font-family: "hack"; }
The following CSS reduces the spacing within the module headers for a more compact module list:
#module-header { padding: 0.15em; }
Before:
After:

