menu font size problem

Hi guys,

I’m trying to change the font size in main nav menu but after the saving the original size cames back. Any ideas?

Thank you
#main-nav ul li {
text-transform: capitalize;
font-family: Oswald,arial,Georgia, serif;
font-size:18px;
position: relative;
display: inline-block;
float: left;
border:1px solid #222222;
border-width:0 0 0 1px;
height:40px;
}

https://wordpress.org/plugins/menu/