Then google comes and do some version of Modern UI on web and Android, which looks and feel miles better than MSFT!
The double border (thick black + thin white) is good for usability/accessibility but it's just ugly and also draws a ton of attention to itself on whatever page it's on.
On the plus side the typical case of design along these trends looks indistinguishable from the sort of thing that used to make people shake their head and go "I see the developer made this UI, we'll send it over to design to make it look nice, typical developer, doesn't get design" which means you can match average modern design quality without... knowing anything. So that's nice. Just claim something design-related in your title or how you introduce yourself or something and people will accept it—they'll still say it's bad if your title and reputation is just as a developer.
[EDIT] for example—look at the "upcoming" for the Button element on the linked site. Yeesh.