You can watch the training video here:
In the Account Dashboard, click Manage button in your project site as shown in the screen below:
Note: You need to create at least one project/site to set the themes for your site.
You will be re-directed to the Project Dashboard. Click Settings on the bottom of the screen as shown below:
As shown in the screen above, click Theme Editor. You will be re-directed to the Theme Editor screen as shown below: