The background-size property specifies the size of the background images.
The size of the image can be fully constrained or only partially in order to
preserve its intrinsic ratio.
If the background-size is contain or cover:
The image is rendered, preserving its intrinsic proportion, at the largest
size contained within, or covering, the background positioning area.
If the image has no intrinsic proportion, then it is rendered at the size of
the background positioning area.
RGBA color values are an extension of RGB color values with an alpha channel – which specifies the opacity for a color. An RGBA color value is specified with: rgba(red, green, blue, alpha). The alpha parameter is a number between 0.0 (fully transparent) and 1.0 (fully opaque).