To make your theme override templates in the openedx ace_common/edx_ace/common directory, you’ll need to put those templates under the lms/templates/ace_common/edx_ace/common directory in your theme. It’s unintuitive, but seems to be the only way the theme will recognize these files.
We had some luck doing this for the Course Update emails, by overriding
openedx schedules/edx_ace/courseupdate/email templates with theme files under lms/templates/schedules/edx_ace/courseupdate/email/.