[Observium] dark ui image legend legibility

Howdy, all!

is there some voodoo necessary to coerce observium to generate graphs with textual legends that aren't black when dark mode is active? it's kinda hard to view black text on dark blue backgrounds :)

I don't see an obvious knob for this, but it's entirely possible I'm just missing the "legend text color" or "make graph text legible in dark mode" or "learn to read" button somewhere :)

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