OOUI was originally created with a classic design for buttons and other
fields, and that theme (now called 'Apex') is still available and
maintained.
https://doc.wikimedia.org/oojs-ui/master/demos/?theme=apex We
could switch to it at a moment's notice. Personally I wouldn't mind seeing
it again ;)
Still, buttons in the default theme are not entirely "flat", they have at
least borders (or strong backgrounds) to distinguish them. The biggest
problem is the existence of 'frameless' buttons (in both themes), which
look just like normal text if they don't have an icon or something.
--
Bartosz Dziewoński