The font colors are not set/controlled through XML. They are hard coded.
The kitchen works with baksmali/smali to make the code edits. Classes.dex is baksmali'd then several edits made across different .smali files to set the colors. The output is then ran through smali to recompile classes.dex which then gets put back into the apk, along with all the graphics changes (depending on what options you choose in the kitchen), the apk is then zipaligned and signed.
There is no "htc_ime theming guide". Look for standard android theming guides as there is really nothing special (aside from hard coded color values) about theming this vs any other APK file. Keep in mind, you will need to properly .9 patch any .9 images you modify or you will end up with a result that was not what you were intending.
You could always grab a themed keyboard from my kitchen and the plain one from Jonasl's thread, baksmali them both and compare the changes in *.smali to see how the colors are being changed. There are, I think, 3 or 4 files that get changed. As far as any of the other images, just look through the apk for the image you want to change and change it.
I am a programmer. I have taken all the hard work out of theming/customizing this via my kitchen. Take the 5 seconds it takes to sign up and either build your own red sox themed keyboard, or start with one of the preset skins.
It's ridiculously easy.
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