Modes, extensions and other GNOME Shell assets are searched in appropriate
subdirectories of each directory in XDG_DATA_DIRS, falling back
to global.datadir.
However, this isn't the case for themes, which are currently always expected
in global.datadir, even when referenced by a mode in a different XDG_DATA_DIR.
The fix is to have the theme finding pattern follow the same logic as other
elements.
Fixes#167.
(cherry picked from commit d6d09fd3c82dd39dac1a764073040b57baeb8659)
da96408098