The theme designer has been improved to allow you to import and export your themes. Once you open the theme designer from the Control Panel or a theme selection panel, you will find two new buttons at the top of the designer page: one for import and one for export.
These buttons load and save a theme from and to your computer respectively. You can use the feature to back-up your themes, or to share your themes with other users. While there are only built-in themes publicly available as of the time being, there are new themes being prepared and these will be soon to be available for you to download.