Custom pseudo-class :--dialog for styling views inside Dialogs

[CatchABus]

Given the PR I'm currently working on #10514, I had the inspiration of including a way to style views that specifically reside inside dialogs.
That's currently a big problem since there's no easy way to style them.

I was thinking we could use Google Chrome's custom pseudo-classes for such scenarios: https://css-tricks.com/custom-state-pseudo-classes-in-chrome/#aa-custom-states
Custom ones differ in the fact that they start with --.

Example:

Button:--dialog {
    color: black;
}

or another alternative implementation

:--dialog {
    color: black;
}

[rigor789]

I like the idea of moving towards web-standards in general. Even for other custom css properties, I'd prefer we slowly moved towards spec-compliance, for example:

.example {
  placeholder-color: red;
}

would become

.example {
  --placeholder-color: red;
}

to stay spec-compliant.