background-image: linear-gradient(to top, #ff0000, #0000ff);
background-image: linear-gradient(to right, #ff0000, #0000ff);
background-image: linear-gradient(to bottom, #ff0000, #0000ff);
background-image: linear-gradient(to left, #ff0000, #0000ff);
background-image: linear-gradient(45deg, #ff0000, #0000ff);
background-image: linear-gradient(-30deg, #ff0000, #0000ff);
...

background-image: linear-gradient(to right, #ff2121, #ffb42a, #d3f100, #00d470, #007bfe, #3000d8, #9e00e2);