How to theme firmware
Since the same beginners questions come up again and again, it's probably a good idea to have a central place that explains it. That's what the "Themeing step by step guide" is good for. It does not go into details, it just describes how to quickly get a new look on your player. I'm not a technical writer, so if you find anything confusing, tell me about it and I'll do my best to make things clearer.

If you want advanced information, read the help file included with H3Mod. There IS a help file. It contains a lot of information. Read it, or I may get snappy if I hear a question that is answered there already ;-)

How to create preview screens
It's very helpful for theme creation if you can see what things will look like without flashing your player every time. That's what the preview is for. But creating these images is a lot of work, and can really only be done by someone who owns the player, and can compare the preview with the screen.

The Preview creation tutorial teaches you how to create your own preview screens. I really can't do those too well without a player for testing, and with the tutorial, there is no more excuses why YOU can't do it ;-)

How does H3Mod work?
I frequently get asked "You hacked the firmware, can you also add support for pdf files, while you're at it", or similar. I haven't. I know bonkers about how the firmware works or how to disassemble it. I only wrote a tool that can find and replace image data in a file. And, based on other peoples' work, it can also de/encrypt some types of firmware.

To give you and idea of what really happens when you theme firmware, have a look at this Insider Tutorial about the inner workings of H3Mod.