Using Underscores for the first time. Can someone advise how I can center the nav horizontally across the screen and not aligned to the left?

<nav id="site-navigation" class="main-navigation" role="navigation">
			<button class="menu-toggle" aria-controls="menu" aria-expanded="false"><?php _e( 'Primary Menu', 'allport-autos' ); ?></button>
			<?php wp_nav_menu( array( 'theme_location' => 'primary' ) ); ?>
		</nav>

/*--------------------------------------------------------------
5.2 Menus
--------------------------------------------------------------*/
.main-navigation {
    position: relative;
    float: left;
    width: 100%;
    display: block;
    clear: both;
    font-family: 'Lato', sans-serif;
    text-transform: uppercase;
    background: #313131;
    background: hsl(0, 0%, 19%);
}
.main-navigation ul {
	list-style: none;
    margin: 0;
    padding-left: 0;
}
.main-navigation li {
    float: left;
    position: relative;
}
.main-navigation a {
    display: block;
    padding: 1.3em 1em;
    font-size: 14px;
    font-size: 1.4rem;
    text-decoration: none;
    line-height: 1.3em;
    color: white;
    color: hsl(0, 0%, 100%);
}
.main-navigation ul ul {
    position: absolute;
    left: 0;
    z-index: 99999;
    display: none;
    float: left;
    padding: 0;
    background: #4d4d4d;
    background: hsl(0, 0%, 30%);
}
.main-navigation ul ul ul {
    left: 100%;
    top: 0;
}
.main-navigation ul ul a {
    width: 200px;
}
.main-navigation ul ul li {
}
.main-navigation li:hover > a {
    color: #fff;
    color: hsl(0, 0%, 100%);
    background: #4d4d4d;
    background: hsl(0, 0%, 30%);
}
.main-navigation ul ul :hover > a {
}
.main-navigation ul ul a:hover {
    background: #313131;
    background: hsl(0, 0%, 19%);
}
.main-navigation ul li:hover > ul {
    display: block;
}
.main-navigation .current_page_item > a,
.main-navigation .current-menu-item > a,
.main-navigation .current_page_item > a:hover,
.main-navigation .current-menu-item > a:hover {
    background: white;
    color: #313131;
    color: hsl(0, 0%, 19%);
}

.main-navigation .current_page_ancestor {
    background: #4d4d4d;
    background: hsl(0, 0%, 30%);
}

.main-navigation ul ul .current_page_parent,
.main-navigation .current_page_parent .current_page_item > a {
    color: #fff;
    color: hsl(0, 0%, 100%);
    background: #313131;
    background: hsl(0, 0%, 19%);
}

Your main navigation is floated left. Remove that line of css. Set its display property to inline-block. Then put a wrapping element around it and set its css with text-align center.

 

This would be far easier to explain if you put it up on jsfiddle or codepen.

just give position:relative,

 

else text-align:center