[Karellen]

I am guessing that those radio buttons are in the Textures.xbt file.

You have obviously found the skin folder, so go to \skin.transparency\media and you will see a file named Textures.xbt

You have to extract it, (then compile it if you make any chages) with this... https://kodi.wiki/view/TextureTool When using this tool, it does not like spaces in the path or filename for itself or the file you are working with, so save it to your root. Extract the xbt file into a folder with no spaces in the name, and look at the images. I guess it will be called something like radiobutton or similar. Make your changes and recompile it.

Now if you want to give yourself options, name the new xbt file something like... DarkBlue.xbt and save it next to the existing default.xbt. Then you can access the new colour scheme from the Settings menu under Themes, same page where you swap skins. (remember to restart Kodi after you add the new files)

If you go to your \skin.transparency\colors folder, you will see the default.xml which stores all your colours. This file and the default.xbt work togethar. If you change the colours there, and save it as DarkBlue.xml (same as the xbt file) when you load the new theme, both files will load together.