You should try adding what you want. It's real simple on OGS. For example, I just added above mentioned color, looks rather good (Named Sienna). As for the stones, it a bit harder since you need a lot of pictures for different sizes, but still nothing stops you from fixing things your way.

I find Magnolia too light. I use SakuyaAkira's Shadows.

As for the stones, I was Gimping for a bit, but I can't remove the border in a way that still looks good.

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a1h1 [1d]: You just need to curse the gods and defend.
Good Go = Shape.
Associação Portuguesa de Go

Yeah, I have added the background "Ochre" which looks fine, but the stones seem to be a bit more complicated since obviously transparency and antialiasing is needed which requires semitransparency.

Yep, I tried editing the simple stones set a bit, and my basic gimp skills are not good enough.

_________________
a1h1 [1d]: You just need to curse the gods and defend.
Good Go = Shape.
Associação Portuguesa de Go

Being able to use custom textures would be great, but I'd put some other features (such as showing game list and chat in different colors) or be able to refresh connection (mine is very bad) ahead of this.

I have created a stone set with GIMP now including alpha-transparency/antialiasing. They look nice, but unfortunately, they cannot be uploaded. The error message says, that either a file is missing, is not in png format or exceeds a 4k-ish size. Actually all 60 stones are entered in the upload text field, they are all in png format, and the size of the largest file is just 774 bytes.

Sigh.