feat(design): update nav list background color to animate via opacity for performance

[xelaint]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/graycoreio/daffodil/blob/develop/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[x] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

Nav list has a background hover style that is being animated through transitions. Background is not a property in the GPU and animating it causes slower performance.

Part of: #2446

What is the new behavior?

Add the background hover color via the after pseudo-selector and animate it via the opacity property.

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information