[EUI+] Fix missing subdued background colors

[weronikaolejniczak]

The purpose of this task is to fix missing subdued background colors in several components in the EUI+.

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Cases:

• EuiPanel’s shaded example doesn’t actually have a shaded/subdued background. This appears to be an issue across much of Docusaurus,
• EuiText’s <pre> and <code> children are missing a subdued background color,
• EuiCard visual errors:
  • EuiCheckableCard’s checkboxes/radios are misaligned with text,
  • Subdued background color is missing.
• EuiCommentList visual errors:
  • Subdued background colors are missing,
  • EuiComment UI breakdown - letters are not correctly vertically aligned with blue circles.
• EuiTimeline multiple items example looks very broken due to Docusaurus theme heading margins and missing subdued background color.