Removed 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 2/3 respectively).

This opens the door for changing header colors and gettings appropriate
changes throughout the theme.
Re-shade menubar and treeview selected items.

This change re-centers the shades used in menubar and treeview items
around the 1.0 mark, so that the overall effect remains the same but
their color is directly determined by @theme_selected_bg_color.
Centralized theme coloring changes.

All text (normal and selected) is rendered as #5f5f5f. Restored the
original selected_bg_color to #accd8a.

Recolored row-selected.png to match the new selected_bg_background
color.

Re-ordered "settings" lines for easier maintenance.
Merge pull request #24 from wrouesnel/remove-manual-color-specifications

Restore original coloring