You can change themes through the settings as shown below.
It would be cool if there was an option to change the theme from a button with a dropdown list of the available themes and have the theme applied to the preview which is then applied to the design file on save.
Thank you for the suggestion! The main toolbar is premium real estate and only features that are used frequently can be displayed there. I am not sure if changing themes clears that bar. Of course we are open to feedback, so if others think there should be a menu entry we would like to hear about it.
If you click the settings button, the first thing you’ll see is the drop-down menu to change the theme. To me it must be a waste of time for the developers to add it. But one extra click is off course more difficult 
