Click on the 'Flex Container', open the Text panel (⌘T) and change the typeface.
I placed my own MP4 files and a YouTube video in my slides and it isn't working.
If you have both YouTube (or Vimeo) and self-hosted videos, you need to have them in different SLIDES. This is necessary because you can only choose one video source for each SLIDE. So, make sure the video source is 'YouTube or Vimeo' on one SLIDE and 'HTML5 Self-Hosted Video' on the other SLIDE.
Do I have to fill up all 20 slides before getting another SLIDES?
Not at all, you can have any amount of slides (less than 20) per SLIDE.
I have a lot of video slides and now my page loads extremely slow.
Under Video Controls > HTML Video you probably have Preload enabled. Preload loads all of your video slides in the background. Disable the preloader feature to speed the loading of your page.
Why is my description text overlapping my video / image in the Pop-Up Lightbox?