Improve on-call schedules colors for dark mode

Gitlab has a dark mode! On-call schedules are very visually color-dependent, so we should take a look at which colors to use for dark mode.

Context

Previous discussion: !54003 (comment 511438570)

There is a different color palette for data visualizations in dark mode; the chevron-skipping pattern is a tiny bit different.

Ideally, we want to:

• Utilize that alternate pattern when dark mode is enabled.
• Switch to white text

Expected level of effort: M