The correct answer is *no* since you can also override templates in a module ([example](http://drupal.stackexchange.com/a/136570/28700)). It doesn't sound like you want to do that, though.

At minimum you would have to create an *.info file that includes the dependency:

    name = My Theme
    description = Just a basic theme
    core = 7.x
    base theme = base_theme_name