Issue with Flash of unformatted content (FOUC)

Hi team
We’re having a problem with QuadMenu (Pro) loading before CSS styles kick in – so there would be maybe half a second of unstyled quadmenu showing and it does not look good.
Would anyone know how to fix this problem?

The URL of our website is https://siteera.com. The problem is visible on pretty much all pages. Thanks!

The page I need help with: [log in to see the link]