@Abland thanks so much for sharing your great workaround!
@TallblokeUK actually, Microthemer is only meant to prevent checkbox selection in targeting mode (when trying to select a checkbox for styling it). So this was a bug which I’ve just released a fix for in version 5.2.1.6. You should be able to update via your WP dashboard now.
Please let me know if you need help with anything else.