[EuiSteps] Remove chroma transparency warning, misc cleanup

[cee-chen]

Summary

Several Kibana devs have reported this chroma.js warning for disabled EuiSteps:

Unfortunately, it looks like #7032 did not fully resolve the console warnings. The usage is coming from here:

eui/src/components/steps/step_number.styles.ts

Lines 72 to 74 in a107059

	color: ${makeHighContrastColor(euiTheme.colors.disabledText)(
	euiButtonFillColor(euiThemeContext, 'disabled').backgroundColor
	)};

euiButtonFillColor(euiThemeContext, 'disabled').backgroundColor is a color with alpha transparency on it, so we cannot call makeHighContrastColor() on it without this warning.

I've opted to revert disabled step numbers to use the same text & background colors as euiButtonFillColor() entirely, while leaving the step title the same higher contrast color. IMO, this is significantly DRYer approach to take - if we need to increase color contrast, we should do so on buttons as well as steps.

Before	After

This PR also DRYs out/cleans up some other CSS, which can be viewed by commit.

QA

• https://eui.elastic.co/pr_7454/#/navigation/steps#horizontal-steps
• Confirm no console errors about contrast and transparency in dev tools
• Confirm zero axe browser plugin violations for low color contrast

General checklist

• Browser QA
  • Checked in both light and dark modes
  • Checked in Chrome, Safari, Edge, and Firefox
  • Checked for accessibility including keyboard-only and screenreader modes
    - [ ] Checked in mobile
• Docs site QA - N/A
• Code quality checklist - N/A, CSS only changes
• Release checklist
  • A changelog entry exists and is marked appropriately.
    - [ ] If applicable, added the breaking change issue label (and filled out the breaking change checklist)
• Designer checklist
  • Updated the Figma library counterpart

[cee-chen]

@1Copenut do you mind weighing in on this since you approved #7032? IMO, the step number is less important in terms of contrast than the step title/text. If we still feel it needs higher contrast, I think we should do so in the euiButtonFillColor and propagate that to all buttons instead of just steps.

[kibanamachine]

Preview staging links for this PR:

• Docs site: https://eui.elastic.co/pr_7454/
• Storybook: https://eui.elastic.co/pr_7454/storybook

[elasticmachine]

💚 Build Succeeded

• Buildkite Build
• Commit: 6b80f3c

[1Copenut]

👍 LGTM! I took a look at the view in light and dark themes and ran the axe browser plugin. Zero issues or console output. Contrast ratio on the gray text was 4.78:1 and 5:26(?):1 on light and dark themes respectively.

Clean refactor @cee-chen!

[cee-chen]

Thanks Trevor!