Rated 5 out of 5 stars

Thank you for introducing this useful feature. Some suggestions for improvement :
- several highlighting colors (as said before)
- removal of highlighting
- contextual menu entries (if possible!)
- keyboard shortcuts

This review is for a previous version of the add-on (1.0). 

Thank you for your suggestions - first I will work on options page for colour choice and then let's see what I can implement ...
Edit 04.12.2020: new version with options page and many colours available.
Edit 07.12.2020: New unmark function in version 3.0: hold "Shift" + click "Mark!"-Button to set background of the selection to white.
Edit 13.12.2020: Quick colour change realised in version 4.0.