[Feature Request] Add customizable theme and color settings.

[WindsorMacmillan]

Is your feature request related to a problem? Please describe.
I expect more theme configurations of custom segments instead of only 3 levels (good, medium and poor). And also bossbar color theme.
Something like:

color-scheme {
#When value (in %) reaches the level below, use that color.
performance-level= [25,50,75,100]
performance-color= [green,yellow,gold,dark_purple]
bossbar-level= [15,30,45,60,75,90]
bossbar-color= [aqua,green,yellow,gold,red,light_purple]
text=gray
text-secondary=white
}