Thought I'd have a look at this last night.
The backgrounds are just images so can be swaped/edited, they are in /usr/lib/enigma2/python/Plugins/Extensions/CoolTVGuide/Cool3D/Menu.
And can create your own 3d button design by editing image files in /usr/lib/enigma2/python/Plugins/Extensions/CoolTVGuide/Cool3D/MyDesign.
Then I got stumped, well gave up for the night as was late
I tried editing a xml file for the CoolTinyGuide to change text to black so would show up on white/grey backgrounds and after uploading my box got the GSD every time I selected the CoolTinyGuide.
At first I though it was a permissions problem so altered permissions but no go.
I was a bit indiscriminate in altering the colour tags though, doubt that would be a problem though?
Just had a thought, used gedit to edit files, that should be ok shouldn't it?
In the CoolTinyGuide.xml could see where in the file creates the header and footer stuff but couldn't see where controlled the main epg stuff, but it was late so will have another look when get chance.
Haven't looked into skin.xml as matrix said as forgot was mentioned and just ploughed in, I'll have a look there next I think.
If I didn't want to change the text colour it would be fairly easy to change the look and feel by changing backgrounds/buttons so have a go anyone is bored lol, If you get the GSD the easiest was to get working again that I've found is to just reinstall, your preferences are stored anyhow from what I've seen. But warning reinstall will wipe any new images you've put in place for buttons/backgrounds.
Anyhow, that's my little update of what I've done/tried to do so far!