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

assets/themes/default/dark.scss

@@ -0,0 +1,12 @@
+/* color palette: https://coolors.co/23f0c7-ef767a-7765e3-6457a6-ffe347 */
+
+@import url('./feather.css');
+@import url('./fonts/Route159/Route159.css');
+@import url('./fonts/Lora/Lora.css');
+@import url('./fonts/Playfair_Display/PlayfairDisplay.css');
+
+@import 'dark_variables';
+@import 'global';
+@import 'header';
+@import 'article';
+@import 'forms';