When it comes to larger projects, first:

- you don't need to understand the whole in order to help

the kind of bugs you can start with are like :

- this icon is a bit weird, it's off centre by 2px - how do I add 2 pixels to this icon? either by moving it or by changing the underlying image asset? if I'm moving it, what is the subroutine that paints it? if I'm changing the image asset itself, where is it stored? (is it in a packed store? or is it just a plain file, etc)

- when I click this button, trace the pathway - it's supposed to add to history and turn blue. is it doing that?

etc.

For large projects, start super small and work your way out from there.