Gem for OneNote v43 improve the "Save Page as PDF" feature to save the page background in PDF file.
Click "Gem" tab -> "New" group -> "Save As" menu -> "Save Page as PDF" menu item.
Gem will start save current OneNote page as PDF file.
After save completely the page to PDF file, Gem will open the PDF file with PDF reader. You will found the PDF file keep the original background color.