How can i make subforums to look like this?

[mirko_sofer]

Example:

This is my subforums:

 

[Splicho]

You can do this with Display: Grid.

.YourSubNodeListClass {
    display: grid;
    grid-template-columns: repeat(auto-fill, minmax(200px, 1fr));
    grid-gap: 1rem;
}