The computer opens the PPT file and clicks "Design" on it. On the design page, click a theme. On the theme page, click the theme type, and then click Insert Use. After the completion, PPT has set the theme. Detailed steps:

1. Open the PPT file and click "Design" on it.

2. On the design page, click a theme.

3. On the theme page, click the theme type.

4. After the completion, PPT has set the theme.