When using a custom theme in the SharePoint site that your Access app is located, you may encounter an issue where all action bar icons display within your popup form.
- Navigate to the SharePoint site that contains your Access app (from within the Access app, click Back to Site).
- Select the Settings icon and choose Change the look
- Select the Office theme, click Try it Out
- Click Yes, keep it
- As desired, Go back to Change the look and reselect your custom theme
The developers have reported that this issue should now be resolved for all tenants.