Theming (#624)

* Theming

- Custom CSS for blogs
- Custom themes for instance
- New dark theme
- UI to choose your instance theme
- Option to disable blog themes if you prefer to only have the instance theme
- UI to choose a blog theme

migrations/postgres/2019-06-19-141114_themes/down.sql

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+-- This file should undo anything in `up.sql`
+ALTER TABLE blogs DROP COLUMN theme;
+ALTER TABLE users DROP COLUMN preferred_theme;
+ALTER TABLE users DROP COLUMN hide_custom_css;

migrations/postgres/2019-06-19-141114_themes/up.sql

@@ -0,0 +1,4 @@
+-- Your SQL goes here
+ALTER TABLE blogs ADD COLUMN theme VARCHAR;
+ALTER TABLE users ADD COLUMN preferred_theme VARCHAR;
+ALTER TABLE users ADD COLUMN hide_custom_css BOOLEAN NOT NULL DEFAULT 'f';