Theme customise

Hi, i’ve try to add a border to my nagigation menu, but the problem is I can’t resize it.
He is my blog iamkady.com
And this is my css :
.site-navigation {
position: absolute;
border-style: double;
border-color: #E6E6E6;
border-image: none;
border-width: 3px 0;
top: 300px;
border-width: 100%;
height: 50px;
float: left;
}

please , help me
Plus, i want to center the menu items