Correct use of FluentOverlay.BackgroundColor

[kakkamstrup]

Maybe I'm not just getting it, but how am I supposed to set the BackgroundColor parameter in FluentOverlay?
The docs tell me to set it like (#rrggbb or #rgb), but as I understand the framework, I should instead be using one of the build in values like var(--neutral-layer-1).
The FluentProgress also seems to work that way with its Color parameter