The GH Remote control panel is not using a WebView.
As someone who’s made Rhino UIs with WebViews (and like doing so) if you are only targeting Rhino for Mac and Windows, I would strongly suggest using ETO. If you look at David’s videos on YouTube, GH2 is using ETO controls for everything. This isn’t to say there isn’t a lot of customization, but you can get your fillet corner buttons in there somehow.
You might get some hints from this thread: Eto Forms Styling - #7 by curtisw