This PR enables `ApplicationHighContrastAdjustment::None`. Doing this
disables a set of mitigations in XAML designed to band-aid apps that
were never explicitly designed for High Contrast (HC) modes. Terminal
now has full control of and responsibility for its appearance in HC
mode. This allows Terminal to look a lot better.
On paper, we should be able to set `HighContrastAdjustment="None"` on
the `<Application>` element. But that doesn't have any effect. I don't
know if this is a bug in `<Toolkit:XamlApplication>` or somewhere else.
So instead I set the property in codebehind, which is not as ideal, but
does at least work. I'd love to a way to move this into App.xaml.
The Find box had a couple stray styles to override the ToggleButton's
foreground color. With backplating removed, these styles became
actively harmful (white foreground on highlight color background), so I
just removed them. The built-in style for ToggleButton is perfect
as-is.
Closes#5360
182a3bb573