style(flat-button): implement inverted tone

[ahmehri]

Summary

Add inverted tone to FlatButton component.

Description

This is needed when placing FlatButton on a dark background which makes the content not visible enough.

[montezume]

Thanks for the PR, but instead of introducing a new prop, I would rather use theming for this case. I'll push a commit to show you what I mean

[montezume]

if it's a dark background, it makes more sense to wrap that background in a ThemeProvider, overriding the colorSolid.

[montezume]

This is what it looks like in the VRT app

[montezume]

This way, if you included other UI-Kit elements that support theming, like text for example, you could do something like this.

<ThemeProvider theme={{colorSurface: 'black', colorSolid: 'white'}}>
    <FlatButton theme='secondary' label="Click me" />
    <Text.Body>Hello world</Text.Body>
</ThemeProvider>

updated pr

[montezume]

I guess we could also add "inverted" prop, with duplicated functionality, but I would like to remove the inverted prop from the other components in the next breaking change release, so I don't really see the point 🤷‍♂