This theme features responsive video embeds. Videos you post will keep the aspect ratio that was given to them by the video hosting (YouTube, Vimeo) at any screen size. Here is an example of a YouTube video:
And here is a Vimeo video:
To insert a video into a post, a page, or a portfolio project, just paste a YouTube or Vimeo video URL. WordPress and the Stills theme will do the rest for you by making sure to keep the aspect ratio and to make the video not overflow the screen area at any screen size.
There are other interesting features in this theme, like galleries playfully alternating between 1- and 2-image rows — see how that works and looks.