Header flashes - loading issue on website

When I load any page on dundurnhairdesign.ca , the header css isn't fully loaded and causes the header and page to flash quickly which isn't appealing for my client. I have built the website alltoolgroup.com and have not run into this issue on that website, as the header does not change at all when the pages load. Any help would be appreciated! I have looked into the preloader plugin but I dont want to go that route as I dont need it with the latter site that I built. I am not sure what is causing this.