From Xojo Documentation
The WebStyle class to apply to the control. A WebStyle can be used to change the appearance of a control.
In Safari, the WebPopupMenu control will not respect a style you set unless that style has a non-white background. This is a Safari limitation. Safari adds its own style to the native controls (popupmenu, checkbox, radio button, etc.) to make them appear more native-looking. This style will override any style you assign unless you change the background to something other than white. Even setting the background to 255, 255, 254 (which is as close to white as you can get without being white) will work.