[AppBar] Make dark by default for dark themes

[cmeeren]

• I have searched the issues of this repository and believe that this is not a duplicate.

Summary 💡

According to the MD spec:

In a dark theme, the surface of the top app bar uses a dark color instead of a primary or secondary color.

I propose that in MUI, when setting a theme's palette.type to 'dark', the App Bar is by default rendered with a suitable dark, neutral color instead of the primary color.

Examples 🌈

Screenshot from MD spec:

Motivation 🔦

This is a good default that follows the MUI spec. Currently it must be done automatically, e.g. by having the theme override the app bar's color prop to 'default', but that sets it to a very dark color that I'm not sure is correct:

[oliviertassinari]

@cmeeren Thank you for opening an issue. Yes, we will have to rework this point for v5 :)