Update header navigation theme colors

What does this MR do?

Here's a short story. Today marks 1 year since I started contributing to GitLab. I've been using the Light theme since day one and this was probably the first user interface issue that I noticed back then and I thought I should give this a try! Thanks everyone! 🏀

Before	After

What are the relevant issue numbers?

Fix #49658 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer