In specific cases when updating from a version transitioning 2.4.0, it has been reported a couple of times that the site appears visually broken.
This happened because the CSS/JS assets refreshing process has not ran itself.
Please try clicking on Rey admin bar menu > Refresh > Assets Cache (Css/JS) ex:

In case you’re running WP Rocket with Critical CSS enabled (Optimized CSS delivery on Async mode), please refresh the Critical CSS too in WPRocket’s settings as well.