Of course themes can depend on another theme, by having the other theme specified in the BaseTheme directive in the Theme.txt file. What's more however, that themes can also have dependencies on modules: just use the Dependencies parameter like with modules in the Module.txt.
Not too complicated, right? Still it's something not obvious. Actually since jQuery is an always-on core module you don't need this, but if you have any other JS library in a module that you use in your theme you have to declare the dependency. You get the idea :-).