Describe the bug
When rendering a FormToggle component outside WordPress, it is displayed with wrong styles because of the lack of the WordPress core styles.

To Reproduce

• Demo: https://mmtr.github.io/wordpress-components/#/form-toggle
• Code: https://github.com/mmtr/wordpress-components/blob/master/src/examples/form-toggle.jsx

Expected behavior
It is styled like if the component is rendered inside WordPress.

Screenshots

Desktop:

• OS: macOS High Sierra
• Browser: Chrome
• Version: 67

Additional context
Issue found while working on #8338 and Automattic/wp-calypso#26367