If I want to install and use a different desktop environment than came with my distro, what (if any) problems can I expect to encounter?
I'm not knowledgeable about kernel tweaks, but switching DEs is very easy on Debian and should not cause any problems. Just install the desktop metapackage (e.g., "task-kde-desktop") and you'll be able to choose it from the login screen.
It's a sample message, Toastify is the JS library used for notification popups. I've seen this a couple times when Hexbear didn't load completely, so I just refreshed it