This article below introduces in detail how to apply a background color in PowerPoint.
Step 1: Go to the Design -> Variants -> Background Styles -> choose one of the preset colors:

Step 2: If you want to select different color schemes -> click on

Step 3: The Format Background window appears, choose Solid fill -> select a color for the slide in the Color section, adjust the parameters in the Transparency section to change the transparency of the background color.

- Click on Apply to All to apply to all Slides -> The result after applying:

- If you do not want to apply the selected background color, click on Reset Background to choose another color for the Slide.
Above is the detailed guide on how to apply a background color in PowerPoint.
Wishing you all success!