Use animated gradient backgrounds in slides and video

Use a PNG when the background should hold still, and an MP4 when it should move. Keep the canvas at 16:9 (the default) so it fills a standard slide.

Still backgrounds

Export a PNG at 2x so it stays sharp on a projector or a large screen. In any slide app, set it as the slide or master background so every slide matches.

Moving backgrounds

1. Keynote: drag the MP4 onto the slide, choose Arrange > Send to Back, and in the Movie panel set Repeat to Loop.
2. PowerPoint: Insert > Video > This Device, then on the Playback tab choose Start Automatically and Loop until Stopped, and send the video to the back.
3. Google Slides: upload the MP4 to Google Drive, then Insert > Video > Google Drive and set it to play automatically. Slides plays a video once, so use a longer clip.

Moving backgrounds work best behind short text. Keep the animation slow, and pick colors with enough contrast for your text. Free videos are 5 seconds; Pro goes to 30.

In a video editor

Drop the MP4 on the bottom track in Premiere Pro, Final Cut Pro, DaVinci Resolve or CapCut, and put your footage or titles above it. For a background longer than one clip, repeat it on the timeline and add a short crossfade where the copies meet.