[Accessibility] Labels on bar charts have dark text on dark colour bar (colour contrast)

[tonypaulbarker]

In testing #173 I found this issue against 2.x

The colour contrast of text directly over the bar charts is not sufficent - I'm not able to read the numbers.

The chart is one chunk of code in the DOM so it's not easy to inspect and find the problem.

To reproduce: Install demo content and visit:

/election/general-election-july-2024/banbury
/election/general-election-july-2024

Screenshot:

[markconroy]

This is the same issue as #147

I'll close the other issue as the title of this one is clearer.