1/ place folder inside your themes folder

2/ at the top of your gtrc file inside the folder you will find a place to name your theme

3/ rename your folder to match your theme name in part 2

4/ further down you will see a place to set your centre padding color currently "#454545"

5/ the images to change are bg.png and table.png

6/ restart your Gimp go to preferences Themes and select your theme

7/ now restart your gimp again and it should start with your theme