[BUGFIX] Improve visibility of last checked row in dark mode

[ennakouriamina-pixel]

The current implementation uses a hardcoded background color (#ace9ff) to highlight the "last checked" row in the broken links table.

In TYPO3 dark mode, this results in poor contrast and readability.

This change:

• Replaces the hardcoded color with a TYPO3 CSS variable for light mode
• Introduces a dedicated background color for dark mode to ensure proper contrast
• Adds support for prefers-color-scheme: dark as a fallback when no explicit theme is set

The highlight now remains clearly visible in both light and dark modes.

Tested in TYPO3 v13 backend:

• Light mode: OK
• Dark mode: OK
• Auto dark mode (prefers-color-scheme): OK

[sypets]

Thank your for your PR. I could reproduce and PR fixes problem.