ToyKeeper’s repository has all the source code.
You’ll need to download all of the relevant .h and .c files, then use something like Atmel Studio to make changes before compiling your own .hex file.
This thread and this thread are good places to start if you haven’t done this before.
I make a few small tweaks to Andúril and compile .hex files for lots of my lights. TK’s code is nicely commented and easy to follow. I can fumble my way through it with very little prior programming experience. 