[Resolved] adding a box sidebar

Sure, with Page Builder, you can create a two-column design. When editing the row click on the right cell, a new Cell Styles panel will appear, you can set the background color via the Cells Styles panel.

When editing the cell in question you can try setting a negative margin in the Attributes > CSS Styles field, something like:

margin-top: -100px;

I can’t say off-hand if that’ll work but you can give it a try and see how it goes.