The scopes are determined by the syntax. You may find it helpful to have a look at the .tmLanguage files of any languages you are working with.
Also, in case you haven't worked it out yet, themes cascade -- like CSS. So make sure you cover all the basic categories (string, keyword, variable, constant, etc.) as defined in the TextMate docs: http://manual.macromates.com/en/languag ... onventions
Hope this helps,