Fix active hover state for navigation section

[aduth]

🛠 Summary of changes

Fixes an issue where an expanded navigation section will show incorrectly with a light background when hovered.

See: 18F/identity-dashboard#748 (comment)

The background color here is meant to apply for hover states within the mobile navigation, and should not apply for larger viewport sizes. The solution is to add a media query to prevent the color from applying beyond the mobile navigation viewport width.

📜 Testing Plan

1. Run npm start
2. Go to http://localhost:4000/header/
3. Click "Section" or "Current Section" to expand navigation items
4. Keep cursor hovered over the navigation item
5. Observe that it shows correctly with a dark-blue background and white foreground text
6. In the "Mobile" preview at the bottom of the page, hover over navigation items
7. Observe that the hover background still shows as light-blue background and dark-blue foreground text

👀 Screenshots

Before	After