resizing the frame of a video

hello there,

so the support team tells me I need custom code and I am a clueless user. 

my problem: my vertical videos are shown only horizontal and i can not change the frame without code. Since I do have horizontal videos on my site as well, it should only work for every single mp4 differently. Desktop and mobile alike.

I attach a screenshot. you do see videos in the screenshot where it worked, this was last year when squarespace did it's job as I thought it would do today again.

And I would be super thankful for telling me how to get into the famous css section. Every time I try this: https://support.squarespace.com/hc/en-us/articles/206545567-Using-the-CSS-Editor my site is automatically going back to "squarespace.com/config/design".

Thank you very much for any ideas!

found the css section in the meantime 🦁