The end user should be able to customize the highlighting by turning on and off (for the different "levels") and changing the appearance. Preferably, an example of what the highlighting looks like should be shown in the preferences. This would probably require some custom Javascript for the preference page. Example of this in the Translate extension: https://github.com/wikimedia/mediawiki-extensions-Translate/search?q=appendFromSelect&unscoped_q=appendFromSelect.
On and off should be easy enough by just adding a checkbox in preferences and the functionality shouldn't be to hard to implement.
Customizing appearance can be done in a few different ways:
- Letting the user change the CSS themselves in commons.css T178270
- Easy/nothing to implement, only requires instructions on how to do so.
- May be to tech heavy for some users.
- Changing colour etc. in preferences, see M169
- Requires overriding the default CSS rules from the preferences.
- Preferably with some kind of fancy colour picker.