Login to the CMS.
On the left-hand side, in the tree menu, click on 'Design'.
On the right side, you will see a grid of your available 'Theme Designs'.
To create a new 'Theme Design', above the grid, click on the button 'Create Design'.
To edit an existing 'Theme Design', find it in the grid and click on it.
If you are creating a new 'Theme Design', in the field at the top, enter a name and then click Save in the bottom right.