[Feature Request] Enable greyscale for background only

# Feature Request

## Is your feature request related to a problem? Please describe

I really like the greyscale option, I work with colour and try to minimise the amount of on-screen colour on other apps, because it influences my perception. Particularly annoying are the almost-but-not-quite neutral grey backgrounds you get in a lot of dark mode themes. BAckgrounds are annoying because they occupy a lot of screen space. However some sites are less useable when greyscale is turned up, because foreground elements, e.g. links or avatar badges are also made greyscale.

## Describe the solution you'd like

It would be a brilliant if there was a way to desaturate all the background-colour and leave all foreground elements in colour.

## Additional context