The free edition of KPT Style for PowerPoint for Mac.
It adds a KPT Style Basic tab to PowerPoint with a Themes gallery of ten palettes, recolour for the objects you have selected (white, black and grey stay as they are, text stays readable) and Scale by % with a preview.
It installs next to KPT Style ADVANCED without touching it: a different file, a different tab and a different folder.
The installer asks for your Mac password because PowerPoint reads add-ins from a folder shared by every user of this Mac. No account, no ads, no tracking, nothing is sent anywhere.