Hi,
It sounds like a FOUC issue. If you are using a heavy cache plugin or any CDN cache which allows lazy loading of stylesheets, that can cause this problem as the delayed loading of the styles will cause the flash.
Check the cache configuration and make sure to disable any lazy/delayed or “via script” loading of stylesheets, it will resolve the problem. Some CDNs may call it “rocket loader” or similar.
I usually don’t recommend using these advanced techniques of loading assets in general, unless you can gather the critical CSS to prevent the style flash. They usually cause more problems.
Let me know if this helps!