Score values sometimes difficult to read in data-viz tooltips

[robyngit]

Tooltips are used to show the scores for a given region in both the rank bar chart (on the region scores page) and the scores globe (on the global scores page). The score value is shown against a background that has the corresponding colour for the score:

When these score colours are dark, the score is difficult to read:

The text with the score value should switch between light/dark text dynamically, depending on the background colour. See for example "How to decide font color in white or black depending on background color?" and "Dynamically changing the text color based on background color contrast with vanilla JS"