It is not so easy (even impossible) to give you the one correct answer.
It depends, every theme has a different answer on this question...

You actually should know a scripting language: VBS, Powershell, PHP, CSS, XML there all different, but you know the basic idea of a scripting language and you find your way...

But can you tell me, what your theme is and what you want to change now, maybe I can give you the right direction...

Grtz,
Bert