RFC: remove theming

[tkh44]

I'm starting to think we should remove built in theming and just let users use the theming library themselves. Having theming built in adds overhead and code complexity to styled that I don't think is necessary. I prefer to just import my theme file but I may be an outlier.

Some notes.
If we remove theming...

• Styled becomes a pure functional component and we can create a factory with createEagerFactory to call it 🔥
• Bundle size will drop
• Performance will be better
• Users that want theming can just wrap styled with withTheme
• We may be able to add theming conditionally with babel option (we could just wrap styled with withTheme and import it if the user wants theming)

[Connorelsea]

I'd be happy with the decrease in bundle size when not using theming. Emotion currently sitting around 3.4kb in my app

[thangngoc89]

@Connorelsea since version 6.0 , you have to opt-in for themeing to use it.

[tkh44]

closed in #225

[ericclemmons]

Personally, I wouldn't mind if theming could be automatically like so:

import styled from "emotion/react/themed";

export default styled.div`...`;

Which would be the equivalent of:

import styled from "emotion/react";
import { withTheme } from "theming";

export default withTheme(styled.div`...`);

Having converted a large styled-components repo to emotion, everything was largely find/replace besides wrapping all of the components with withTheme.