[#9076] [Eslint] Fix: Allow CSS keywords in no-css-color ESLint rule

[kapral18]

Closes #9076

Summary

Fixes false positive warnings in the @elastic/eui/no-css-color ESLint rule that incorrectly flagged valid CSS keywords as literal color values.

Problem

The rule was triggering warnings for CSS declarations like:

• border: none
• border: 0
• color: inherit
• background: transparent

These are valid, theme-agnostic CSS keywords that shouldn't require EUI theme colors.

Root Cause

When parsing CSS like border: none, the cssstyle package fills in default values per CSS spec:

module.exports.initialValues = new Map([
  ["border-width", "medium"],
  ["border-style", "none"],
  ["border-color", "currentcolor"]
]);

The rule was checking if (Boolean(style.borderColor)) which returned true for "currentcolor", even though it's a CSS keyword, not an actual color literal.

Solution

Added a whitelist of CSS keywords that should be allowed:

• currentcolor - Inherits the computed color value
• transparent - Fully transparent color
• inherit - Inherits from parent
• initial - CSS initial value
• unset - Removes property
• revert - Reverts to browser default
• revert-layer - Reverts to cascade layer

These keywords are:

• ✅ Semantically correct (not actual "colors")
• ✅ Theme-agnostic (work in light/dark modes)
• ✅ Part of CSS specification

Testing

Test Coverage

• ✅ Added 9 new test cases for CSS keywords
• ✅ All 119 tests passing
• ✅ Verified in Kibana codebase

Before Fix (False Positives) in Kibana

// ❌ Incorrectly flagged
badge: css`
  border: none;  // Warning: Avoid using a literal CSS color value, use an EUI theme color instead [@elastic/eui/no-css-color]
`;

After Fix

// ✅ No warning
badge: css`
  border: none;  // CSS keyword - allowed
`;

// ⚠️ Still correctly flags actual colors
badge: css`
  border: 1px solid red;  // Avoid using a literal CSS color value, use an EUI theme color instead [@elastic/eui/no-css-color]
`

[elasticmachine]

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[weronikaolejniczak]

Tests are passing (we don't run them in CI 😬 soon...), the code changes look clean and I like the comments above the test cases (makes it immediately clear what the test is about). Thank you for contributing, @kapral18! 💪🏻