Hi UPukaJRm, thank you, I appreciate it! Custom shortcuts for custom markers is a good idea - will be implemented with next release (which will take a while, because I'm currently working on a bunch of new features).
For deleting highlights I recommend using the context menu (right click), or in case you want to delete the last highlight it would be safer to use the corresponding shortcut for "Undo", which you can customize as well.
For deletion of all highlights I'm not so sure if that would be something that is really needed, because you could just delete the whole entry from the history, which has the same effect;) Feel free to email me to share your thoughts on that (undflybir at gmx.de); alternatively you can open an issue on github: https://github.com/ufb/Textmarker/issues

UPDATE: changing degree of appreciation of this review from 5 stars to no stars. If you encounter a bug you can contact the developer via email or via opening an issue on github, but trying to manipulate me with the number of pentagram icons won't work and is not appreciated.