[Mac] Fixed bug with Popover where the TextEntry is gray.
[Mac] Fixed bug with Popover where the TextEntry is gray.

* There is is a bug on AppKit where if you don't set the `Appearance` for you view, the `NSTextField` will be displayed with gray background

https://stackoverflow.com/questions/29074724/how-to-change-nstextfield-background-color-in-nspopover

antmicro/xwt,hwthomas/xwt,lytico/xwt,TheBrainTech/xwt,mono/xwt