Open the Command Palette and run Preferences: Color Theme.
Select Cupertino Day or Cupertino Night. Or one of the colourblind versions!
If Cupertino Night or any of the colourblind versions does not appear immediately after installing a new VSIX, run Developer: Reload Window once and reopen the color theme picker.
Package
Install the packaging tool:
npm install -D @vscode/vsce
Create a VSIX package:
npx vsce package
Publish
Create a publisher in Visual Studio Marketplace.
Login with vsce.
Publish with:
npx vsce publish
Install
Grab the latest version from the vscode marketplace, or use the download link here. If using the downloaded version, go to the extentions tab, click the 3 dots at the top(next to refresh), and click Install from VSIX.