Provide better documentation for withTheme

[enricopolanski]

I feel like the documentation for withTheme is pretty dry and would benefit from having a better description for what the feature does, what problems does it solve and would benefit from having an example code.

• This is a v1.x issue.
• I have searched the issues of this repository and believe that this is not a duplicate.

[oliviertassinari]

@VulcanRav What extra information would you like to know?
https://material-ui.com/customization/themes/#withtheme-component-component

[enricopolanski]

I sincerely do not understand what's the purpose of this method or when/how to use it.

I'd guess it's used to access the theme variable in a component I wrap with this HOC, but so far I've been unable to access it as a prop (which is the point of making an example in the docs) and looking at the code and tests I'm still confused. It seems like the theme is now in the HOC's state, but I'm still not sure how to use it (I guess with state.theme). But is the theme here passed from a parent MuiThemeProvider such as in style functions when passing the theme as a prop?

[oliviertassinari]

@VulcanRav Alright, we need a more detailed example. This HOC injects the theme property into the component so it can be used in the render method.