Add how-to for scss compilation

themes/learn2-dark/README.md

@@ -8,3 +8,13 @@ A custom dark theme based on Learn2
 
 ## SCSS Watching
 To compile scss into css, install the package `ruby-sass`  
+
+```sh
+scss --watch scss:css-compiled
+```
+
+For one-time executions:  
+
+```sh
+scss --update scss:css-compiled
+```