Question Use custom skin with Plesk Premium Email

[webthingz]

Server operating system version

ubuntu 22.04

Plesk version and microupdate number

18.0.60

Hi There, I'm thinking of buying a custom skin that I like and have a question about it.
I'm using a Plesk server with Plesk Premium Email extension.

When a user has premium email enabled the selected skin is set to the default skin for plesk premium email.
(tested with the litecube-f skin from roundcube plus)

So I would like to know if it's possible to change the defalt skin of Premium Email and I can't find the path where this skin lives.

Hope somebody can help me with this.