In the Theme task pane, click New theme colors or New theme effects.
Note: If you don’t see the Theme task pane, on the Format menu, click Theme.
In the New Theme Colors or New Theme Effects dialog box, choose the colors or effects you want, and click OK.
Tip: Before you create a custom theme, first apply a theme that is similar to the theme you want to create. Then, when you open the New Theme Colors or New Theme Effects dialog box, most of your choices will already have been made.