Fix colors in search results page footer

[atsansone]

From https://buganizer.corp.google.com/issues/337297674

Observed Results

Having changed the CSS to forced colors, observe in the search results page that the search button is not displayed properly also the footer.

• Screenshot macOS
• Screenshot ChromeOS)
• Screenshot ChromeOS

Expected Results

Ensure that the search button and the footer should be displayed with all styles and also the icon.

User Impact

Low vision users are affected because the search button and the footer are not displayed accordingly.

Steps to Reproduce

1. Enabled the CSS forced colors.
2. Open the following page: https://docs.flutter.dev/search?q=devtools
3. Continue the linear navigation using the Tab key.
4. Observe that the search icon has disappeared.
5. Also check the footer and the styles are disappeared

GAR Assessment

Flutter Docs

GAR Details

1.16. (d) With forced-colors (encountered by way of High Contrast Black on Windows)

Environment Details

Machine: MAC/ChromeBook Browser: Chrome Version 120.0.6099.272 (Official Build) (64-bit) Screen reader: NA. Bug filling time: 15 mins.