It's not possible to save a Form in Form Builder, trying to close Form Builder discards the draft.

Steps to reproduce:
1. Login as admin
2. Go to Content -> Forms
3. Start creating a new Form
4. Open Form Builder (click the "Building form" button)
5. Add a button and configure it
6. Click the "X" in the top left cornet

Expected:
I'm back to Content Edit view, form preview is displayed

Actual:
Clicking "X" discards the draft

See screenshot to see how it looks and how it should look - looks like there is a CSS issue there.