dude, don't give up so easily, use this as an opportunity to learn some image editting - which you will need to do when you get round to making your own skins. I's like anything else, a bit daunting when you are completely new to it, but you slowly pick things up. If you make backups first then you have nothing to lose by experimenting.
dunno if you knew but you can find the themes/bmps in the sysmetrix installation directory - usually C:\Program files\Sysmetrix\Themes
give it a whirl m8, you might be surprised at what you can do - good luck