In some of Kit's sections like Promo (08) there is an option to include a background video. For these videos, you need to essentially provide a URL to an MP4 Video file.
Thinkific makes it a breeze by utilizing their course "download" lesson.
Create a Web Optimized MP4 video file. You should always limit the length of your video to 30-40 seconds at the most, or even better, find a short video that loops seamlessly, so that it seems like a longer video, but could be less than 1MB or 2MB in size, depending on the length. Videoblocks is a great resource for these videos.
Create a dummy course inside your Thinkific site
Create a download lesson and upload the video file
Open the course as a student
Right click on the download link and copy it
Paste it into the Kit section settings
Also create a Thumbnail for the video
When you are using a background video, you should also provide a "poster" image. This image will be used on mobile devices that do note support HTML5 video and also will be shown briefly before the video plays.
Play the video in full screen mode on your laptop or desktop
Pause the video on frame one
Capture the screen
Crop this image in Canva so that it is 1920x1280
(Alternatively) use this black image as the "poster" image so that the first frame is always black.