How to make Upfront Lightbox responsive?

Hi there,

I tried and searched quite a bit but cannot find the solution.

I want to display a video inside a lightbox which opens upon clicking a button. (see screenshots). It works but only on desktop view, on mobile or tablet the size of the lightbox and the video inside are way too big. Editing the size for mobile in responsive mode did not do any good, as it changes the size of the lightbox even for the other screen sizes as well.

How do I make the lightbox behave responsive?

Many thanks for your support! I really appreciate it.