This Karta bankowa tył icon is made in Material Design style developed by Google. This style is based on extremely simplified shapes, thin two-pixel lines, and is optimized for 24x24 px. Unlike other vector icon packs that have merely hundreds of icons, this icon pack contains 4,300 icons, all in the same style and quality.
It's a small rectangle with rounded edges. There is a black bar that crosses the entire rectangle a centimeter from the top edge. There is a smaller line underneath the bar that is set slightly in from the left hand side and ends at the middle of the rectangle.