I do not understand.

Maybe do not see it as techie stuff, but as configuration.

User styles are a way of configuring your browser to display individual pages differently. Just as you can tell it to do things for all pages in the normal settings of the browser. Like what search engine to use or how big the page should display.

Some sites also have configuration with cookies or account settings. Like using a dark theme or continuous scroll.

For ignoring a user, I suggest install browser extension "Stylus" 

And then configure a style for use on itch. Replace username1 with the username you want to ignore, as it is written in the url. This will hide all thumbnails to projects of that user.

@-moz-document domain("itch.io") {
div.game_cell:has(a[href*="//username1.itch"] ) { display: none }
div.game_cell:has(a[href*="//username2.itch"] ) { display: none }
}