Customization with custom.scss documentation (#56)

* Customization with custom.scss documentation

* Update Customizing.md

Running-Mastodon/Customizing.md

@@ -0,0 +1,20 @@
+Customizing your instance
+=========================
+
+## Customizing style
+
+If you create `app/assets/stylesheets/custom.scss`, the default css will be replaced by the content in `custom.scss`.
+
+## Changing colors
+
+If you want to customize for example the vibrant color of your mastodon instance, you can put the following code in your
+`custom.scss` file :
+
+````scss
+$color4: #d3d900; // vibrant
+
+@import 'application';
+````
+
+Don't forget to recompile your assets and restart mastodon(if you didn't have a `custom.scss` file before) 
+to see the changes.