Guide for saving a gradient background on Keynote 14.4 master slide

I am trying to create a master slide in Keynote version 14.4. The official Apple Keynote guide does not seem to address this accurately for this version. Can anyone tell me step-to-step what to do to save the gradient I create for the background of a master slide? Or point me to an authoritative and accurate "sole source of truth?"

[Re-Titled by Moderator]

Original Title: keynote master slides