Take a look in the theme's images folder for:
These are the two images you have been advised to edit.
If these images are present in the theme's image folder they can override the Theme development/tweaker settings
An alternative solution you can try is to rename the two files as backup copies:
- tcp_bg.png-BAK
- tcp_bgsel.png-BAK
REAPER will not see the image files, so the color choices in the Theme development/tweaker dialog will have an effect and you will not need to edit the png files.
.