x/website/tour: use system setting prefers-color-scheme

[Deleplace]

Since 509116, the Tour has a dark mode, which is awesome.

However, it shows up in light mode by default, even if my OS/browser settings say I prefer dark.

The rest of the website has had the same problem at first, and it has been fixed. The user may click a top-right icon to toggle the light/dark theme, and this explicitl user choice is persisted. However, the default theme (without any user action) should acknowledge the system-wide preference.

[Deleplace]

cc @jamalc

[hyangah]

Indeed this is confusing. x/tour is a separate codebase, so not handled in the recent accessibility improvement work. Moved to the unplanned milestone.