It looks like you added some code in the wrong place.
If you look around line 266 in your style.css file you’ll see this –
.menu-primary li,
.menu-secondary li,
float: right;
#header .menu li {
float: left;
list-style-type: none;
}
You added float: right; in the middle of the elements when it should be in between the { } right along with the others. Once you move or remove it then it should return back to normal…..and of course you can always replace the float: left with float: right if you want.