鈥s they indicate an active selection similar to radio or check buttons. See Tweaks > Windows for example.

I think this comes directly from Adwaita, where check marks are black in check buttons as well, while in Communitheme they are white with green background. I wonder if they would be less visible if green, though, since is a lighter color
Yes, checkmarks should green! Following the guide by Canonical.

Does anybody know from where this check mark comes from? It's not the one under gtk-3.0/assets
I'm not sure where the icon is from. Maybe a fallback icon from the Adwaita theme? It's present in GNOME Software as well.
Either way, the checkmark in GNOME Tweaks isn't a "real checkbox" like the others that use the green, hence the different icon and black color. I feel bad for saying it since I have no clue how to contribute to GNOME Tweaks, but I don't think it was implemented well. Maybe it should be reported as a bug?
It can be fixed, but I don't know if we should bother:
#ListBoxTweakGroup #Focus box ~ image { color: $success_color }
I just summoned Jeremy Bicha, GNOME Tweaks upstream, pointing him to this thread so that you can discuss the issue :)
It is object-select-symbolic. I believe you'll find it's also used in gnome-control-center and several other GNOME apps.
In the GTK-CSS, these check marks are just image, not check, and usually don't have a specific style class. Therefore, we cannot change the color of all check marks exactly. So I think this can be closed as wont fix.
Thank you @nana-4
closing this
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I just summoned Jeremy Bicha, GNOME Tweaks upstream, pointing him to this thread so that you can discuss the issue :)