Click right mouse button or equivalent while cursor is over a highlighted passage. There should be a menu item that says "This highlighted passage...", with options for your choice of modifying (basically changing the colors) or removing the highlight.

I had to make this a separate menu from the add-on's own context menu because WebExtensions API doesn't offer a SelectorContext for context menus. HTML does, so I got devious and told my content script to attach such a menu to each highlighted element.