We just shipped a new theme to go along with "default", "night theme", and "pink theme". It's called "minimal light".
It uses more soft grays and whites than the default light theme and is less busy overall. Enjoy!
I already see a couple places where the colors need to be adjusted now that it is in prod, but that will be worked out shortly.
We currently have a dedicated set of themes, and don't allow full-on customization, because when things change we sometimes need to update all the themes. I'm not sure the interface would ever be so stable as to allow for arbitrary themes.
Any thoughts for additional themes? I feel like a "sunset theme" with subtle oranges could be nice. I could see a situation where we could allow folks to go from light to sunset to night themes automatically depending on time of day.
Reminder: you can also switch between serif and sans-serif fonts in /settings/misc.