Remove all manual color references from theme bodies.

This commit replaces all explicit color references (e.g. #FFFFFF) with
versions derived from the theme colors specific in the relevant header
files (settings.ini and gtkrc for GTK2/3 respectively).

This opens the door for changing header colors and getting appropriate
changes throughout the theme.

Its an architectural change to the files which matches all original theming
colors exactly, but makes maintenance and widespread changes much
easier.