Here's how you can set up Borders and Corners on a page section using the Advanced Tab (hover).
1) In an OP3 page builder, select a section you would want to make edits.
2) Go to Advanced Settings (cogwheel icon) >> Advanced Options >> Hover section.
3) Use the dropdown arrow at the Border & Corners tab to display available options.
4) Start making edits on the available options.
5) Don't forget to save changes once done.