Press F5 to open a new window with your extension loaded.
Open Code > Preferences > Color Theme and pick the "Spectrum" theme.
Make change to the /themes/spectrum-vscode-theme.json file.
UI : For all changes to the "outer UI", like (status bar, file navigation etc.), take a look at the Theme Color reference.
Syntax: For changes to the "code highlighting", examine the syntax scopes by invoking the Developer: Inspect Editor Tokens and Scopes command from the Command Palette (Ctrl+Shift+P or Cmd+Shift+P on Mac) in the Extension Development Host window.
Commit your changes and open a PR.
Note: Changes to the theme files are automatically applied to the Extension Development Host window, so no reloading should be necessary.