If you are on an oe firmware (except for 3.10 oe), then go to recovery mode, select 'advanced', and select 'Toggle USB (flash0)'. Connect your psp to a pc and go into vsh>resource. Put your new topmenu_plugin.rco in there and overwrite the old one. Change the name of 'background.bmp' to '01-12.bmp' and place it in the same folder and overwrite. Go out of recovery mode and toggle the USB as you normally would through the XMB. Place the 'wallpaper.jpg' in PSP>PHOTO and disconnect the usb. Go into photo through the XMB and select the image. Press triangle and choose the 'set as wallpaper' option.
Hope that helped.
PS. Vibestar, these themes are amazing