Add Foliage theme and make it new default

Moment should have a different default theme than Mirage to make it more
visible that it's not the same software.

docs/THEMING.md

@@ -4,14 +4,14 @@ A default theme from this repository can be copied to use as a base and edit,
 for example:
 
 ```sh
-    cp mirage/src/themes/Midnight.qpl \
+    cp mirage/src/themes/Foliage.qpl \
        "${XDG_DATA_HOME:-$HOME/.local/share}/mirage/themes/MyTheme.qpl"
 ```
 
 Or for Flatpak users:
 
 ```sh
-    cp mirage/src/themes/Midnight.qpl \
+    cp mirage/src/themes/Foliage.qpl \
        ~/.var/app/io.github.mirukana.mirage/data/mirage/themes/MyTheme.qpl
 ```