Here is the response from the theme group. They were not aware of the issue, but gave a simple workaround.

You are the first to report that problem. We will investigate to find a solution.

Meanwhile, here is a little trick:

Click on "Ctrl and -" one or two times to zoom out in your browser.

You should be able to see the bottom of the customization window.

To zoom in, click "Ctrl and +" or "Ctrl and 0" to reset to 100%.

Hi Bruce,

The problem with posting your question here is that very few of us actually have the Clean Theme.  Only the 250+ supporters of the campaign.  You may find a faster solution emailing odoo at design@odoo.com.

If that does not work then I would recommend posting a GitHub Issue but only after you have tried contacting them directly. 

I do hope you will update your question here when you find the solution.  And best of luck with your problem.