Added $color[15] for the "unselectable folders" color.

Added $color[15] for the "unselectable folders" color.
Made left_main.php use $color[15] and fall back on $color[6] if not specified.
Added a $color[15] for all themes.  If you don't like the new color, just
change it.  :-)
Fixed a bug in prefs.php.
Updated themes.txt documentation.

git-svn-id: https://svn.code.sf.net/p/squirrelmail/code/trunk/squirrelmail@1991 7612ce4b-ef26-0410-bec9-ea0150e637f0