When using the Theme option in the Toolbar and saving changes, you may encounter the following error:
Try navigating to admin > Design tab > Advanced Editor and look for any colors that may not have color number listed, or that appear blank, such as:
In the above example, click 'Site Backgrounds' which appears below the Apply to Site button to get to the above options.
Replace any transparent options with a color, such as #ffffff and click 'apply to site'.
Test your Theme changes once more and see if this time it is able to complete the change.