Open Theme button on the toolbar and select the theme file to load.
Double click a theme file
You can double click a .theme file in a folder or in a Visual Studio project to load it in Theme Builder.
Select a preloaded theme
Every time you load a theme in the Theme Builder, it's added to the list of preloaded themes. You can switch easily between themes by selecting from the drop down.
The full path of the current theme is displayed in the status bar at the bottom left of the main window.