Fix low contrast on downloads graph labels and borders

[Turbo87]

Resolves #8675

Unfortunately Chart.js does not natively support any kind of dark mode switching (see chartjs/Chart.js#9214), so we have to implement it ourselves.

The first commit adds a resolvedScheme property to the colorScheme service, which is automatically updated when either the user explicitly switches the color scheme, or the user is on the "System" setting and the OS decides that it's time for a color scheme change.

The second commit then uses {{did-update ...}} to adjust the Chart.js settings and run chart.update() when the resolvedScheme property changes.

Light

Dark (before)

Dark (after)