Press F5 to open a new window with your extension loaded.
Open File > Preferences > Color Themes and pick your 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 Developer: Inspect Editor Tokens and Scopes command from the Command Palette (Ctrl+Shift+P or Cmd+Shift+P on Mac) .