Describe the bug
When rendering a DateTimePicker component outside WordPress, the buttons for going to the previous or to the next month are wrongly styled.

To Reproduce

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

Expected behavior
Buttons for going to the previous or the next month are 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