Provide higher contrast marker labels for host applications set to dark mode

[nononoah]

Warning: I have not yet verified if this is reproducible outside of my test application.

When testing in host applications with a user interface style of dark, it appears that accessibility snapshots produce rather low contrast marker labels.

In light mode:

In dark mode:

Ideally, we'd use a text color which is better guaranteed to contrast.

[NickEntin]

Ahh good find! I always run test host applications in light mode, so totally missed it was defaulting to a dynamic color. Thanks for the bug report!