Coordinate text becomes white-on-white on HeatPlotMap in dark mode

When a web page is in dark mode, hovering over a coordinate just shows an empty whitebox, presumably because text defaults to white when switching to dark mode, whilst the bubble stays with a white background.

This behaviour is present in (at least) HeatPlotMap.

Locate the HeatPlotMap component and its hover coordinate display, then inspect the styling applied when the page is in dark mode. Reproduce the white-on-white tooltip and trace which text and background colors it uses. Done means coordinate text remains readable while hovering in dark mode.

Written by the indexing model from the issue text.