Categories sidebar huge empty space

Can we see a link to the issue?

Sure.

https://www.travel-world.eu/

Do you have a section in the dashboard named, “Custom CSS” or “Edit CSS”?

If you don’t have either, install this Custom CSS Manager plugin and use its section of the dashboard to hold this code:

.has-sub-menu:nth-child(2n) {
 float: right;
}

I’afraid this theme has 20+ css files.

Ok, i was able to find the correct css (in uploads/) and add the code above.

Seems to work fine.

Many thanks Andrew.