Concerning the compatibility, there shouldn't be a difference between LMDE and Debian. The choice of the Desktop environment depends on your personal favour. As Mac user, you probably tend towards Gnome which can be installed during the setup of Debian, not LMDE. However, as I don't know about the performance of the 2013 iMac, maybe Cinnamon or Mate are a better choice.
I just installed the Flatpak version of Inkscape (the first GTK application, that came into my mind, where it makes sense to have the latest version installed) and the Breeze dark theme works for me.