Rewrite Theming docs as Customize section

- Broke up existing theming page into separate pages
- Audited and updated each new docs page (some pages include some TBD sections)
- Update sidenav to reflect changes
- Fix links that pointed to the old Theming page
- Update docs styles to reflect recent changes
- Rewrite some bits in the Migration page

site/data/sidebar.yml

@@ -5,13 +5,21 @@
     - title: Contents
     - title: Browsers & devices
     - title: JavaScript
-    - title: Theming
     - title: Build tools
-    # - title: Best practices # TODO: Write this content
     - title: Webpack
     - title: Accessibility
     - title: RFS
 
+- title: Customize
+  pages:
+    - title: Overview
+    - title: Sass
+    - title: Options
+    - title: Color
+    - title: Components
+    - title: CSS variables
+    - title: Optimize
+
 - title: Layout
   pages:
     - title: Breakpoints