You can change the design by easy one click.
From Quick Menu
Go to Switch Design to choose from the list of skins. Your downloaded skins will also appear in the list.
From Submenu Bar
You can also change your design from 
Change Design in Submenu bars.
From Settings Bar
Go to View > Toolbars and select Settings bar to display it.
Downloading New Skins:
Go to the Skin Center and click download button for the skin you like. That's it! You don't even need to restart Lunascape to apply a new skin.
Creating Own Design:
You can create your own skin and share that with other users at the Skin Design Center.
Please download an Instruction to create your skin. Also, it is recommended to take a look at the Cool design file in the Lunascape install folder as an example. That should make the job easy.