It appears your boilerplate is suffering from the style-jsx not being rendered on the server causing a flash of unstyled content (even though most of your content is styled due to the CSS file).
I started an issue on this matter here: https://github.com/zeit/next.js/issues/3434. You might want to check over there for a solution. Or if you get it figured out maybe you could also post over there.
It appears your boilerplate is suffering from the style-jsx not being rendered on the server causing a flash of unstyled content (even though most of your content is styled due to the CSS file).
I started an issue on this matter here: https://github.com/zeit/next.js/issues/3434. You might want to check over there for a solution. Or if you get it figured out maybe you could also post over there.