Hi,

I've heard a few times that compared to say OSX desktop, KDE looks less professional and more like a school project. In particular, you can see lots of sparseness/blanks in the theme, and the windows/dialogs are usually too large relative to the amount of information they contain.

I'd love to help improving the theme, but don't know where to start. I'm completely new to KDE development. Is there source code somewhere on Github or something like that so I can modify and make a pull request?

Thank you very much.

I'd suggest that the first place you should look is over in the Visual Design Group sub-forum where they are discussing just these kinds of things. We are always welcoming of new people, whatever your skill set; and seeing that you seem to have some coding experience that may be put to good use making the actual improvements, your assistance will be most welcome.
Before you get too far in coding up your design ideas, I would suggest that you propose your changes in the Visual Design Group sub-forum to make sure that your changes meet the design expectations already established, as they have already discussed and scrutinized most of the details that it appears you want to change.