To quickly test something, you can also override this (or any other) theme in your personal config file. Please follow the guide in the color theme documentation.
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.