Can I switch to EMs or REMs instead of Pixels?

Yes, the software handles it great.

If you want a div with 100vh for example, drag a DIV component in and then go to the "Look & Feel" menu, top right and enter 100vh in the "Height" tab.

If you want to change a paragraph to 2rem, again, go to the "Look & Feel" menu and enter 2rem in the "Font Size" tab