@dln949 The libertine settings app is somewhat fragile at the moment. (Improvement suggestions and PR's are welcomed.)
... an application like mousepad appeared to be installed, but I cannot find the icon for it in the "app drawer".
After installing an application in a Libertine container, it is sometimes necessary to refresh the app drawer to see the new icons. You can do this by swiping down in the opened app drawer.
if I tap on the plus sign and select "Choose Debian package to install", I get this message: "No Debian packages available".
This is the expected behavior. That option is for installing downloaded deb packages. It displays any .deb files that are present in the Downloads folder.
If you know the exact name of the package you want to install, you can use the second button.
I almost exclusively use the third button to search the repositories for the package I am looking for.
But be careful, one stray tap will have you installing an unintended app. And the best course of action is to let the install run its course, before deleting the package and trying again.