Make sure you’re running the latest Usermin, and make sure it has been restarted.
You may need to force a restart by manually killing the Usermin process and starting it again. We’ve had some reports of this issue, and it seems to be due to the Usermin process not being restarted after an update that changed the theme library (which happened with 1.730 and also 1.732).
If that doesn’t fix it, force a theme upgrade in Webmin in the Theme Configuration page, and then restart Usermin. That should make sure all the latest theme files are being loaded by both Webmin and Usermin.