More polished light theme (#7620)

* Revert "Remove pointer to light theme until it is more polished (#7594)"

This reverts commit f8cf85db3b.

* True up the hierarchical structure of mastodon-light theme

* Fix mastodon-light theme

config/themes.yml

@@ -1,2 +1,3 @@
 default: styles/application.scss
 contrast: styles/contrast.scss
+mastodon-light: styles/mastodon-light.scss