activating a theme – do stuff on ctivation

we all know how to ctivate a theme – I hope – but what about doing stuff after a theme loads?

For example, you get a new theme, you accidently, for what ever readon deleted the cutom folder and files inside. How would you, upon activation say – do stuff.

if activated, do stuff. – the stuff I have done, its checking if theme is activated ot not.

Also Does a theme, when updated via the repository (assuming your theme is stored there) deactivate during update process then reactivate? if not what steps do i need to take to say “after upgrade do this…”

Both of these only need to happen ONCE.