Hey there,
This is the english support forum, you can check some specific subforums for other languages(German).
German Support Forum
The language-specific forums are not always very active, so here’s my attempt at helping out, using the magic of Google Translate:
I’d like to know how I can change the style CSS so that when I insert a gallery in the twenty twenty template, it displays four columns of images instead of three. I’ve already checked the WP Content folder on the server. There was a CSS file there, but there was no code there, just comments. My question is, how can I implement this? Could someone please give me a hint as to where I can find the style CSS? Thanks. Best regards
First, to add custom CSS to a website running a classic theme like Twenty Twenty, go to APPEARANCE -> CUSTOMIZE -> ADDITIONAL CSS. Here you can add custom CSS to override any styles you’re trying to customise. You should not edit your theme’s files, because you’ll lose the changes when you update the theme.
That said, you don’t even need any custom CSS to change the number of columns in the WordPress Gallery.
If you’re using the native block editor (aka Gutenberg), you can easily select the number of columns when you select the Gallery block itself. See screenshot below (click to see a larger image).

And if you’re using the Classic editor, you can also easily change the number of columns on the Gallery popup. See the second screenshot.

We are very active in the German support forum 😉
There is also documentation for the Gallery Block that describes the column options: https://wordpress.org/documentation/article/gallery-block/#for-the-gallery-block-2