Creator: Use Twenty Twenty-One styles in editor

[ryelle]

Fixes #388. This adds the same theme styles to the editor as are used in the preview. This is most obvious in button styles, but the font was also different.

Screenshots

Before:

After:

Frontend (see #416 for the alignment issue):

How to test the changes in this Pull Request:

1. Create a new pattern, use headings, buttons, etc
2. View the finished pattern on the frontend
3. The fonts and styles should be the same as in the editor