ui: Theme lookup should respect XDG_DATA_DIRS
mods, extensions and others GNOME Shell assets lookup into each dir, relative to XDG_DATA_DIRS. However, this isn't the case for themes (which can be referenced by a mod in a different XDG_DATA_DIR), hardcoding global.datadir.
The fix is to have the theme finding pattern following the same logic than other elements. Fixes #167 (closed).