How to replace button color

-- HikaShop version -- : 3.1.0
-- Joomla version -- : 3.6.5
-- PHP version -- : 5.6

How it is possibile to replace the buton color (see file attached).
It is enough also one color (not overlay) but I need gray color, not blue.

P.S. Why Paypal button has not background color?

bootstrap.css

Line: 5872