Color contrast on dark theme show invisible link labels

[nickofthyme]

Describe the bug
When using the default background color on the dark theme, or black background, the linkLabel on a partition chart should be white or at least visible.

photo link

To Reproduce
Steps to reproduce the behavior:

1. Go to 24_linked_label.story.tsx
2. Apply no textColor to the linkLabel so it uses the default value
3. Change theme to dark (not eui-dark)
4. Notice the color contrast logic is bad rendering invisible (black) link labels
5. Changing the background color to black causes the same result.

Expected behaviour
Color contrast logic returns a sensible color with black background.