Enable dark/light mode controls (#940)

## Description

Enable dark/light mode controls.

This also reads the system preferences.

## Type of change

<!--- Please check the type of change your PR introduces: --->
- [x] 🗺️ Documentation


## Test Plan

<!-- How will this be tested prior to merging.-->
- [x] 💪 Manual

docs/docusaurus.config.js

@@ -130,8 +130,8 @@ const config = {
       },
       colorMode: {
         defaultMode: 'dark',
-        disableSwitch: true,
-        respectPrefersColorScheme: false,
+        disableSwitch: false,
+        respectPrefersColorScheme: true,
       },
 
       zoom: {