Center the Logo ?

I’m trying to align my logo in the center of my zuni theme but I really can’t manage to do it :( Even if i read a lot of stuff here. I don’t have the premium version, just the free one from wordpress. I read that it’s possible to center the logo with CSS but I tried several things and I really don’t get it. I even don’t know where to put the CSS code.
Could someone help me please?
Thank you very much!