Bug fix in contrastRatio function

[mridulgogia]

Closes #2916
Fix of ContrastRatio function.

Checklist

• Check against all themes for compatibility in both light and dark modes
• Checked in mobile
• Checked in IE11 and Firefox
• Props have proper autodocs
• Added documentation examples
• Added or updated jest tests
• Checked for breaking changes and labeled appropriately
• Checked for accessibility including keyboard-only and screenreader modes
• A changelog entry exists and is marked appropriately

[kibanamachine]

Since this is a community submitted pull request, a Jenkins build has not been kicked off automatically. Can an Elastic organization member please verify the contents of this patch and then kick off a build manually?

[cchaos]

tldr; The SASS contrast calculations do seem to be working correctly now in dark mode. 👍 But someone better at math than me should review the actual code.

Testing this locally by using the makeHighContrastColor() for creating dark color contrast values, this PR does seem to fix the issues we were having before.

Examples:

Also, testing a color combination that fails minimum contrast without a calculation:

When debugging the SASS contrast values:
Before

// SASS Original contrast
contrastRatio($euiColorDanger, $euiColorLightShade): 3.45674
// SASS Calculated contrast
contrastRatio($euiColorDangerText, $euiColorLightShade): 4.50783
// JS Calculated contrast
5.4 !== 4.5

After

// SASS Original contrast
contrastRatio($euiColorDanger, $euiColorLightShade): 4.15215
// SASS Calculated contrast
contrastRatio($euiColorDangerText, $euiColorLightShade): 4.55297
// JS Calculated contrast
4.6 ~= 4.55

[chandlerprall]

SASS code looks like it was ported correctly from the MDN JavaScript example, but I have one question on the first conditional

[chandlerprall]

Code changes LGTM

[snide]

I double checked this one as well. Merging! Thanks for the PR.