You can create or change your own custom skin into our theme easily as following,
Get into the skins folder from your theme directory, where we’ve added our default skin folders with their skin names. The skins folder can you usually found in this path wp-content/themes/designthemes/skins/
Just copy and paste any of them, and then rename it with the new skin name you’re about to be added.
Then you shall edit the “style.css” of new custom skin you’ve replicated. (Just like the images, color, background and border color hex codes with your new custom skin color as needed).
Now you can find you new custom skin folder (created) on your Dashboard > Appearance > Designtheme Option > Skins. Finally you can choose your new custom skin from that settings and click save changes.