The main thing this is trying to communicate is that your CSS files should come above / before any Javascript files. When the browser downloads your HTML, it starts at the top and goes down and downloads resources (CSS, JS, fonts, images, etc).
When the browser comes to a JS file, it calls the server to retreive the file, and once it's downloaded, the browser begins to execute that javascript. When that JS is executing, it's delaying the handling of CSS files which help paint the elements and make them look how you want.
Move your CSS files to be above all your JS files and see if that fixes it.
In general, most (not always ALL) JS files should be loaded as the last thing so that it doesn't block / delay the browser in rendering the page.
