Simi,
I tried to make something to help you understand a little bit better how to code a cover page in Viki tool but I prefer to warn you, I’m not well-versed in this domain 
A big thank you to Justine_desmoulins for her cover that is used as an example in videos and Anna79_9 for her help in “Source” coding. 
Also, this website with codes is really good to test and learn codes you might want to use, or codes that you will see in videos are well explained on this website too:
https://www.w3schools.com/cssref/css3_pr_background.asp
(Quality is not really good, full screen is better to watch them)
The google doc with the Source code if you want to test it or even modify to your liking: https://docs.google.com/document/d/1fiXAXen6f_UkI7JHJN-bfR1SCwkgyJ5juwVkERhIIfg/edit
Google doc: https://docs.google.com/document/d/1ZlzD_D4UCHicDikxE0Q_esPMNLuUljE-_3OpjbD1EjQ/edit