gallery css in Travelify

hello,

I finally found the way to align multiple images on the same line with the gallery.

but where can I find the css of the gallery to make some changes such as margins, borders, … ?

This is not how it works.

First of all you should use WordPress Child Theme Child Themes « WordPress Codex
From there you can use either Chrome or Firefox developer tools to inspect elements you want to style and add this custom code to Child Theme style.css that will overwrite parent theme and WordPress default styling.

This will make your website/theme update safe as this theme is still in development and more versions are expected to be available over the time.