Add chapter: theming a base theme

docs/source/theming/index.md

@@ -19,4 +19,5 @@ theming-engine
 theming-strategy
 custom-styling
 using-third-party-themes
+theming-a-base-theme
 ```

docs/source/theming/theming-a-base-theme.md

@@ -0,0 +1,36 @@
+---
+html_meta:
+  "description": "Customize a base theme like `Volto Light Theme` via SCSS."
+  "property=og:description": "Customize a base theme like `Volto Light Theme` via SCSS."
+  "property=og:title": "Theming a base theme like `Volto Light Theme`"
+  "keywords": "Volto, Plone, frontend, React, themes, Volto Light Theme"
+---
+
+# Theming a base theme like `Volto Light Theme`
+
+In your Volto addon, create a `theme` folder with the follwing two empty files:
+
+```
+src/
+├── components
+├── index.js
+└── theme
+    ├── _main.scss
+    └── _variables.scss
+```
+
+## Variables
+`_variables.scss`
+
+This is the place to override SCSS variables of the base theme.
+
+```
+$text-color: #000;
+$font-size: 18px;
+$line-height: 24px;
+```
+
+## Main
+`_main.scss`
+
+All custom styles should done here. You can also include other SCSS or CSS files here.