support for custom background/stone colors

[popz73]

Throwing in something wilder: how about having a go at supporting custom background and stone colors?

in similar fashion to what OGS does: no storage needed, just a field where to paste an url to the background. stones/grid color would be just a color picker.

[MathieuDreano]

It looks like something could work on. Two options, add it in "theme" or create a second option "board theme" and rename "theme" to "light/dark mode"