Hi Rafaella, thank you for you patience.
I would recommend to see our documentation on how
apply themes and also to create a
Custom Themes.
And regarding the SDK, you can see here how to
include styles.
Also I would like to mention, you are right that applying custom CSS is a risky process future-wise.
Basically, you are free to modify everything you want without any restriction, but there's no other supported way, and we can't guarantee that the custom styling won't break with future updates.
But, unless you change really specific things, or we make really big changes (which we usually don't), everything will be fine.
I hope this helps.