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).
Attica was created using the Theme Studio for VSCode. The Theme Studio for VS Code lets you easily design, and deploy your own VS Code themes from scratch. If you want to create your own fully customized theme I highly recommend checking it out.