Drag the dots on the preview, or use the sliders.
CSS
background-color: #e8f1fb; background-image: radial-gradient(at 8% 12%, #2875c5 0px, transparent 55%), radial-gradient(at 88% 8%, #7dd3fc 0px, transparent 50%), radial-gradient(at 78% 86%, #a7f3d0 0px, transparent 55%), radial-gradient(at 14% 92%, #c4b5fd 0px, transparent 50%);
Positions are percentages of the box, so the same CSS looks a little different on a tall phone than on a wide screen. The phone preview shows how it reflows.
Free Mesh Gradient Generator by Hiyaly