Make sure each pic is in 24-bit Bitmap and there should be:
1. bckgrd_1.bmp
2. bckgrd_2.bmp
3. icons.bmp
4. logo.bmp
5. theme.ini
Now to replacing existing themes...
1. _system_
2. themes
3. random theme folder
4. drag all contents I previously mention and replace those files
5. start up the R4 and in the menu screen (3 icons) press select until you find yours