fix(labs): Fix theming implementation

[anicholls]

Summary

There were two issues with #272:

• During review, we switched from using deepmerge to lodash/merge. I didn't realize at the time that lodash/merge mutates the object it's returning, so we were updating the defaultCanvasTheme unintentionally
• I tried using our own ThemeProvider to mitigate an issue with @storybook/addon-knobs. After realizing this wasn't causing the problem with the addon, I left it as is. However, there is an issue with this in conjunction with our custom styled function and how React hooks work. Since we expect theme to be passed to useTheme through styled from Emotion (since we can't use useContext within class components), we do need to use Emotion's ThemeContext. This is because Emotion is calling our interpolations and passing the theme through (using their context, without the option to override). If we ever move to functional components exclusively in the future, we can change this to use a custom context. Otherwise our current approach with a wrapping styled function will not work with a custom context.

Checklist

• branch has been rebased on the latest master commit
• yarn test passes
• all (dev)dependencies that the module needs is added to its package.json
• code has been documented and, if applicable, usage described in README.md

[cypress]

---

Test summary

56 • 0 • 0 • 0

---

Run details

Project	canvas-kit
Status	Passed
Commit	eb2a61f
Started	Dec 12, 2019 8:00 PM
Ended	Dec 12, 2019 8:01 PM
Duration	01:15 💡
OS	Linux Ubuntu Linux - 16.04
Browser	Electron 61

View run in Cypress Dashboard ➡️

---

This comment has been generated by cypress-bot as a result of this project's GitHub integration settings. You can manage this integration in this project's settings in the Cypress Dashboard

[lychyi]

It may be worth writing a simple test but let's get this into master first.