Hi YOOZU. This is usually done by using CSS. I highly recommend to you, to learn at least the basics of it. It is really easy! You can override the CSS of you template with it, at least to a certain extend.

To add you custom CSS you must go to your website and be logged in. Here you search for "HTML/CSS Editor" or "HTML/CSS/JS Editor" depending on your version of Odoo, within the menu.

In the top left of the overlaying editor, you choose "SCSS (CSS)" from the drop down. In the editor you can try something like this: 

#top_menu .nav-item {
    margin: 0px auto;
}

But be aware that your changes have to work on the desktop, tablet and mobile. 

I also recommend you learn the basics of HTML(5), CSS and how to use tools like the developer tools of Firefox or Chrome. There awesome crash courses on YouTube for example.
Good luck :)