Press F5 (Debug > Start Debugging )to open a new window with your extension loaded.
Open File > Preferences > Color Themes and pick the »Relaxed« 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 Commmand Palette (Ctrl+Shift+P or Cmd+Shift+P on Mac) .
You can relaunch the extension from the debug toolbar after making changes to the files listed above.
You can also reload (Ctrl+R or Cmd+R on Mac) the VS Code window with your extension to load your changes.
When editing workbench colors, it's easiest to test the colors in the settings under workbench.colorCustomizations and workbench.tokenColorCustomizations. When done, run the Generate Color Theme From Current Settings command to generate an updated content for the color theme definition file.
Adopt your theme to Visual Studio Code
The token colorization is done based on standard TextMate themes. Colors are matched against one or more scopes.
To learn more about scopes and how they're used, check out the theme documentation.