Centering Nav without responsiveness changing

Hi, I am having trouble centering the navigation bar, without it messing up the responsive aspect. It looks great until I get it on my iphone, or reduce the browser window size, then instead of the drop down menu for nav items, it squishes them together and shoves them behind my slider. Thanks in advance!

Site is http://www.jmarissa.com

Here is the css code I tried…Kept it on so you can see the problem!

#access ul {
    display: table;
    margin: 0 auto;
}

#site-logo {
    display: table;
    float: none;
    margin: 0 auto;
}