The upload page, video edit page, and private video recipient view page are in light mode by default. This gives those pages a white background with dark text.
In the lower right corner of these pages is a button allowing you to switch to dark mode. Clicking this would switch the page to having a dark background with light text. Clicking it again will switch it back.
Whichever mode you select will remain persistent for all of your video edit pages and private video recipient view pages for you. Your setting will not impact the view setting for other team members or recipients, who can adjust their own appearance setting. Dark mode is not currently available on other pages on Vimeo.