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/_variables.scss

@@ -0,0 +1,29 @@
+// Color Scheme
+$gray: #F3F3F3;
+$black: #242424;
+$white: #F8F8F8;
+$purple: #7765E3;
+$lightpurple: #c2bbee;
+$red: #E92F2F;
+$yellow: #ffe347;
+$green: #23f0c7;
+
+$background: $white;
+$form-input-background: white;
+$form-input-border: $black;
+$text-color: $black;
+$primary: $purple;
+$primary-text-color: $white; // text color on primary background (buttons for instance)
+$success-color: $green;
+
+// Dimensions
+
+$article-width: 70ch;
+$horizontal-margin: 20%;
+$margin: 0 $horizontal-margin;
+
+// Fonts
+
+$route159: "Route159", serif;
+$playfair: "Playfair Display", serif;
+$lora: "Lora", serif;