Making new theme with small css changements but can't find opencart.css and don't know how to override its css

Hello,

Until here no problems but now I want to change the bottom margin of "<div class="swiper-viewport">...</div>" from 60px to 30px.
It is the block where a banner is displayed. With the "inspect elements" function I found next css that seems to be in a file "opencart.css":
".swiper-viewport {
margin: 0 0 60px;
background: #fff;
z-index: 1;
border: 4px solid #fff;
border-radius: 4px;
box-shadow: 0 1px 4px rgba(0,0,0,.2);
width: 100%;
position: relative;
overflow: visible;
}"

Changing this in "stylesheet.css" of my custom theme doesn't change anything.
"Opencart.css" is not in the default theme as far as I could see and further I don't where to look for it.
If someone can localize it, how can I add overriding css?
Or maybe it doesn't even to be localized, just tell me how to override css that is not in the default theme.

hi,

Hope this can help: https://www.youtube.com/watch?v=mqTstomXZiM

Cheers

Hi,
It's here: You can override it in your stylesheet like this:

Thanks. It' the "!important" that made the changement.

Also thanks for the youtube link that I can check out if I haven't forgotten to copy any file I need.