You mean: https://docs.mendix.com/refguide/moving-from-atlas-2-to-3. If you have no or little modifications to your styling, it is not a big task to migrate. If you have lots of styling, it might be an option to add a link to your Atlas2-styling in theme/web/main.scss, but that is not the recommended way.
So far I like the extra possibilities that the new set up provides: you can add styling to a module now.
Just one extra tip: when having a project originally based on Atlas 2 (even when created for Mendix Pro 9), your settings.json is likely to have “cssFiles”: ["styles/web/css/main.css"]
This needs changing to include "theme.compiled.css" for your themsource style modifications being picked up by the Mendix Pro 9 and compiled into CSS.
for example to:
"cssFiles": [ "theme.compiled.css", "styles/web/css/main.css" ],