You can change the design by easy one click.
Go to Switch Design to choose from the list of skins. Your downloaded skins will also appear in the list.
You can also change your design from Change Design in Submenu bars.
Go to View > Toolbars and select Settings bar to display it.
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.