lets see... hex editing. i have a little experience, but not anywhere as near as much as zingas. if your doing some blind hex editing a game, then you have a lot of trial and error ahead of you. for the most part, hex editing a game would be changing already-known values in the code to another value. for example: changing the car color or speed. im sure people already know which line to edit, so that's going to be relatively straightforward. like for GTA:VCS. the values youd have to change for car color and things like that are already known, so you just plug in the values you like and your on your way.

as far as theme making goes, unless your a super theme maker, your going to be editing offsets, not discovering them. so youll really be opening the file, and then changing the line that you read to change in the readme.

you can't write a game in hex, because its kinda hard to read. to actually write a game, your going to need to know C++ or another computer language, and compile it after coding.

Hope this helps