Creating Themes
To create a new custom theme, right click the application and navigate through Themes --> Create New

In the prompt dialog, enter a new name for your theme. (Avoid special characters and names of themes which already exist). Click OK to accept the new theme name.
You are presented with a blank window. To begin the design process, right click the application and navigate through Themes --> Design Active. (Alternatively, press F2 on your keyboard). The designer window appears.

For more information about designing themes, click here