Fix CSS for dark/light mode to conform to accessibility/contrast guidelines

[srvanderplas]

This element has insufficient contrast at this conformance level. Expected a contrast ratio of at least 4.5:1, but text in this element has a contrast ratio of 1.88:1. Recommendation: change text colour to #000201.
Success Criterion:
1.4.3: Contrast (Minimum)
Suggested Techniques:
G18
Code Snippet
...llout-icon no-icon">

Demonstrations