right click, choose Sysmetrix configuration, find your skin in the 'Theme Browser' section, click "Load Theme".
If the skin doesn't apply correctly: Click on 'Theme Settings'. On the right hand side at the bottom there is a text box for "Background". Click on the browse button on the right side of this text box and navigate to the folder where the sysmetrix skin resides (it should be there when you click the button). Then choose the .bmp file for the theme. It is supposed to be called whatever the theme is called. (eg.: Sammy Large.bmp.) When you click on the correct .bmp, your theme will look correct. Click "Save Current Theme" and then "Exit".
The problem is that the folder is supposed to have the exact same name as the theme's background .bmp file. When it doesn't, the theme won't load correctly.
Please let me know if this helped.