Sickday

That looks like the Platinum9 theme for XFCE. Setup for it are basically the normal steps to getting Window Decorations installed for XFCE (here), but in general it’s just:

$ git clone https://github.com/grassmunk/Platinum9 ~/.themes/Platinum9

Then set the Window Decoration theme using xfce4 settings

This is the first I’ve seen of simplelogin and I think I’m going to start using this. Thanks for the share.

Can’t bring your own domains with Posteo.

From their howto site:

Can I use a third-party email app like Thunderbird? No, this is not possible for security reasons. Tuta does not support the use of third-party email clients or the protocols IMAP/POP3/SMTP as we cannot guarantee end-to-end encryption of your data. Instead, Tuta offers email desktop clients for Linux, macOS and Windows as well as a web client and apps for Android and iOS. We aim to provide all required functionalities with our own apps, for instance, offline access to your encrypted data is possible with our apps.