Update color preview shades to match header color

What does this MR do and why?

Updates the shades of the preview plates for Gray and Light Gray themes to match the header color

Screenshots or screen recordings

😆 Personally I can't see the difference in before/after screenshots but they ARE differnt

Before	After

🕶 Dark mode plate also does not match the header color but if changed to match it would be of the same color as for the Gray theme.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #365472 (closed)