fix dark mode contrast for `--destructive` and any other variables

[vincanger]

The --destructive CSS variable suffers from the same problem we had on Demo App's TaskCard component.
The red is not too visible on the dark mode.

To fix the core of the issue, we should change the dark mode --destructive CCS variable to be a bit brighter.
If we do so we can also revert to using destructive color instead of red-500 on TaskCard.

@fgrgic