Possible to create background colour in blocks with curved corners?

I'll start a new thread for this - tks

Ah you are in the page editor > layout, that's not what I meant.

Please read this article from the beginning, it's at appearance > elements > add new > layout element.
https://docs.generatepress.com/article/layout-element-overview/#content-1

Have sorted - thanks Ying.

Glad to hear that :)

You are welcome!