The Popup button object is a Cocoa object. For Cocoa objects, only Apple’s “official” style is allowed. You cannot change the color, background color, text color (or size or font).
The upside is that when Apple changes the appearance of various objects (as they did when Big Sur was released), your Panorama objects will automatically change to match the operating system you are using.
“… changing the appearance …” can mean changing the size. Changing the size means that if you are scrunched down to the minimum for your text, and the button size gets smaller … Same issue for the real estate around your button should the next version be larger.
Best not to design to extremes (unless everything else automatically grows/shrinks as required).