In Mathematica, customizing Sections, Subsections, Titles and so on, can be done with Format > Style > .... Changing the background color of a style is one of the things you can do. Here is an example where I have colored the background orange:
But I want to change the background to show a color gradient. I mean, instead of having the same orange in all of the background, there should be a gradient so the orange fades from top to bottom. Gradient styles will be found in Palettes > Color Schemes. But I can't get it to work for changing the color of a cell background.