Here is the rendaku plugin. [Userscript] WaniKani Rendaku Information
It’s based on the tofugu article about rendaku. It probably covers most cases but of course it isn’t foolproof.
Also, is it possible for you to fix how far the background extends on the color highlighting. As you can see on the app, the background hits the character following the highlighted area making it a little hard to read sometimes. This is especially true for the black background text since the following character will be black as well.
Thanks