Hi Victor-
Generally speaking, this type of “flicker” is known as Layout Shift, or the amount your page changes as the various CSS and JavaScript resources are downloaded. Because Custom CSS is added to the very bottom of your pages, it can be one of the last things to load, and might affect the layout shift on slower connections.
That said, I checked out your site and ran through a couple of pageloads on a slower 3G connection, and I’m not seeing a lot of layout shift. You can check Google PageSpeed Insights, it looks like your layout shift is pretty decent- 0.022 in my test.
Keep in mind that you’re running WP Super Cache, which disables the caching for logged-in users by default, so if you’re checking out your site while logged in, it might be slower for you than for your visitors. I recommend checking your site in an private browsing or “incognito” window so that you can see what it’s like for your visitors who aren’t logged in.