Set a linear gradient as the background image, with linear-gradient() CSS function. You can try to run the following code to implement linear-gradient() function in CSS
Example
<!DOCTYPE html>
<html>
<head>
<style>
#demo {
height: 200px;
background: linear-gradient(green, orange, maroon);
}
</style>
</head>
<body>
<p>Setting background as linear gradient.</p>
<div id = "demo"></div>
</body>
</html>