Set indicator dot colors and button colors based on background color

[heinrichreimer]

We should set the grey dot color always the color of the status bar and change the white dot color to grey similar to the way the status bar switches between light mode/dark mode based on the luminance of the background.
That would make it easier for devs implementing material-intro to rely on readability without having to deal with foreground colors at all.

Possible fix for #37