Centralized theme coloring changes.

All text (normal and selected) is rendered as #2c2c2c. Restored the
original selected_bg_color to #accd8a.

Re-ordered "settings" lines for easier maintenance.
Synchronize theme settings lines and color-shades.

Re-order the settings lines for GTK2 and 3 so the colors appear in the
same order to make maintenance easier.

Match up the GTK2/3 colors exactly.

Foreground selected text is left as the current white-shade.