admin/setting/themesettingmore

In the color fields you can put things like this if you want to get fancy:

   whitesmoke
   #0404E6
   rgb(0,255,244)
   rgba(0,255,244,0.5)
   transparent
   currentColor
   inherit
   hsl(207,38%,47%)
   hsla(207,38%,47%,0.8)

See Standard_themes#Customising_the_.27More.27_theme for further customizations and example