The platform of a chart is the container of the other chart elements, such as the paper, legend, and labels.
To format the platform of a chart:
Click the color image button and select a color swatch to fill the platform (you can also directly type the hexadecimal value of the desired color in the text box). If the color swatches cannot meet your requirement, click More Colors to open the Pick a color dialog, with which you can set a full range of color. If required, click More Fill Effects to specify a gradient, a texture or an image as the fill effect in the Fill Effects dialog.
See also Format Platform dialog for detailed explanation about options in the dialog.