Added Dual box-shadow colour for .skiplink

[lucascumsille]

Fixes: https://github.com/mysociety/societyworks/issues/4213

This should improve the accessibility contrast of the .skip-link element against the navigation bar.

Part of TFL audit. This should improve the following criteria:

https://www.w3.org/WAI/WCAG21/Understanding/focus-visible

[Skip changelog]

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 82.60%. Comparing base (44674bb) to head (7d8180c).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #4977 +/- ## ======================================= Coverage 82.60% 82.60% ======================================= Files 393 393 Lines 30718 30718 Branches 4866 4866 ======================================= Hits 25374 25374 Misses 3895 3895 Partials 1449 1449 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.