fix: increase contrast for replay buttons

[JoshuaKGoldberg]

Requirements

• Filling out the template is required. Any pull request that does not include enough information to be reviewed in a timely manner may be closed at the maintainers' discretion.
• All new code requires tests to ensure against regressions.

Description of the Change

Adds the increased visual emphasis as mentioned in #4911.

Light mode	Dark mode

Alternate Designs

n/a

Why should this be in core?

n/a

Benefits

Users can more easily see the replay buttons.

Possible Drawbacks

n/a

Applicable issues

• Fixes Inaccessible filter/replay buttons in browser reporter #4911
• Patch, bugfix

[juergba]

@JoshuaKGoldberg thank you.
IMO the result looks worse than before with the arrows being white now.

[JoshuaKGoldberg]

Shoot, sorry @juergba - I switched them from black to #fff last minute, forgetting that it's #000 for black 🤦. Updated now.

[juergba]

@JoshuaKGoldberg the support for dark color scheme has been added recently. Please rebase and solve the conflicts, thanks.

[juergba]

@JoshuaKGoldberg thank you for this PR.

[coveralls]

Coverage remained the same at 94.326% when pulling 0518abb on JoshuaKGoldberg:replay-button-accessibility into 41567df on mochajs:master.