Press F5 to open a new window with the extension loaded
Open File > Preferences > Color Themes and select this color theme
Open a file that has a language associated. The languages' configured grammar will tokenize the text and assign 'scopes' to the tokens. To examine these scopes, invoke the Inspect TM Scopes command from the Command Palette (Ctrl+Shift+P or Cmd+Shift+P on Mac)
To start using your extension with Visual Studio Code copy it into the <user home>/.vscode/extensions folder and restart Code.