Vantage keeps overriding styles

Hi,

I am trying to reduce the padding on header and increase the left and right padding on the menu but the theme won’t accept the changes it doesn’t matter if I use custom CSS or the customize theme it still won’t accept it.

header#masthead .hgroup {
zoom: 1;
position: relative;
padding-top: 10px;
padding-bottom: 10px;
}

.main-navigation ul li a {
padding: 15px 60px;
-ms-box-sizing: border-box;
-moz-box-sizing: border-box;
-webkit-box-sizing: border-box;
box-sizing: border-box;
}

Thanks,