Various examples are incorrectly using `DARK_GRAY`

[rparrett]

Bevy version

main, after #12163

What went wrong

Notably, overflow and probably a few other UI examples.

Prior to 12163, DARK_GRAY was a dark gray. However, our css palette now conforms to the css4 spec. In the css4 spec, dark gray is lighter than gray.

This is causing very low contrast in some of these examples.

We should search for usages of DARK_GR[EA]Y and replace them with a darker gray.

This is the old one:

pub const DARK_GRAY: LegacyColor = LegacyColor::rgb(0.25, 0.25, 0.25);

There is no good replacement gray in the palette, so I recommend using Color::rgb(0.25, 0.25, 0.25).

[alice-i-cecile]

See #12176. I don't particularly care what the new colors are for now: just solve the functional issues of low contrast :)

[rparrett]

There's at least one example where the new color looks better IMO: display_and_visibility.