SysMetrix

First, you have to unzip the skin to the SysMetrix themes folder. Usually that's C:\Program Files\SysMetrix\Themes. Make sure it unzips into its own folder within the themes directory. Next, just open Sysmetrix, right click on the tray icon, and choose SysMetrix Configuration. Then go to the Theme Browser and choose the skin you want.

It's also important to make the theme folder the exact same name as the background image. If the background image is called mytheme.bmp then the folder for that should be called mytheme. If it's my theme.bmp, the the folder should be my theme. If you apply a new theme, and it doesn't look at all as is should, you have to go into the configuration menu > theme settings and manually add the background image and then save the theme.