[Styles] expose getContrastText method

[numToStr]

Motivation 🔦

Suppose, the app has 3 different types of background-color like twitter, by exposing getContrastText method it can really help to make custom themes. Currently, there is no way of calculating contrasting text.primary color when creating a theme with createMuiTheme

We can get getContrastText by calling useTheme hook. But I suppose It can be used standalone without calling the hook just like lighten, darken, getContrastRatio methods.

[oliviertassinari]

The method depends on the theme to behave correctly (have a look at the implementation to see the dependencies). I would suggest two possible alternatives. 1. You can start from a lower level helper: getContrastRatio that is standalone. 2. You can make your theme global and import is directly where needed.

[numToStr]

I can't find about getContrastRatio in the docs.

[oliviertassinari]

Yeah, we have an open issue about adding documention for our color helpers.

[oliviertassinari]

#13039

[dantman]

I just ran into/discovered this issue because I need to use getContrastText in an override style, which means the theme object does not exist yet. But I suppose this suggests that overrides may need an overhaul.