Use theme-color
https://developer.mozilla.org/en-US/docs/Web/HTML/Element/meta/name/theme-color

I think --bg_overlays CSS variable should be used.

<img src="https://i.ibb.co/9yh3p7B/Screenshot-20200815-195902-Browser.png" alt="Before using theme-color" title="Before using theme-color" width="300"> <img src="https://i.ibb.co/xgDxyCT/Screenshot-20200815-195839-Browser.png" alt="After using theme-color" width="300" title="Before using theme-color">

I used for the after screenshot, in general.nim, buildHtml:
```meta(name="theme-color", content="#0C0C0C")``` and black theme in settings.