Better Custom Theme integration
As it is, Plesk allows you to install a custom theme that you code yourself "after" you coded it, which is not a good idea.
Once you install a custom theme and relogin to Plesk, there is no indication whatsoever that Plesk is using a custom theme. This is not ideal.
Plesk should have a custom theme selection on the dashboard but, since it doesn't have it, at least an INDICATION that it's using a customized theme.