Buying (and tweaking) a new theme


I'm planning to buy a professional theme for my Omeka project. My initial thoughts of "tweaking here and there", seems -unfortunately- to be a huge project to undertake since I must read and understand both theme files (omeka default and the new one to buy).

I work alone at the moment, no team, no programmers. I understand the programming involved pretty well but still, the work to be done seems a mountain.

So, what would you do? Where should I start from?