Caption under thumbnail adjust margin

2 Jahre 6 Monate her #15213 von Eric Meeuwsen

Caption under thumbnail adjust margin wurde erstellt von Eric Meeuwsen

I'have just started with SIGE Joomla-4 pro and I'm writing a new website.
The temporary version is here: foto.ericmeeuwsen.de/schwarzwald.html
My questionis: how can I get the captions unter the thumbnails to be placed somewhat closer to the thumbnails, that is somewhat higher?

I would use a css like this:
/* Caption under thumbnails in SIGE closer to thumbnails */
.sige_thumb img{
margin: -5px 5px -5px -5px;
}

but I don't know where to place it.
In media/plugins............Asset/Css I only see this file: sige.min.css
Can I put my code in a separate file, whitch will be read as well, such as user.css or custom.css?

Thanks,
Eric Meeuwsen