How to Use a Solid (or Transparent) Color for the Slide Background
On the Design tab, click the Background Styles button.
Background Styles drop-down list opens.
Choose the Format Background command at the bottom of the list.
The Format Background dialog box opens. You can also open this dialog box by right-clicking a slide and choosing Format Background.
Click the Color Picker button and choose a color on the drop-down list.
The muted theme colors are recommended because they look better in the background, but you can select a standard color or click the More Colors button and select a color in the Colors dialog box.
Drag the Transparency slider if you want a “bleached out” color rather than a slide color.
At 0% transparency, you get a solid color; at 100%, you get no color at all.