Fixes override of environment by config files

Config files now have a lower priority than than the environment, so if BPGALLERY_THEME is set in the config, and you declare it on the command line, it does the right thing (previously it ignored the version on the command line). Also locked the functions for the default theme so that other theme files can not overload them.

git-archimport-id: arch@sommitrealweird.co.uk--2005-desktop/bpgallery--mainline--1.0--patch-6