Page Builder changing Font Styling from Canvas
To set styles for use in Page Builder you'll need to navigate to Canvas > Settings > General Styling > Typography
Here you can set your H1 - H6 styles.
Some users have found that using em as an alternative to px can cause Page Builder to not honour the original Canvas styling. This is because em, being a relative unit is effected by font-size of its parent. If this issue is affecting your website simply change 'em' to 'px'.