Jetpack Mobile Theme Issue

I am using Jetpack Mobile Theme and it works great. I am customizing it using .theme-mobile in css. My problem is that whenever I add in the following code it works but also truncates the white background of my site so it goes to grey immediately after this code. What am I doing wrong. I am basically using the div to create a line

CSS

div.spacerline {
width: 90%;
height: 1px;
background: #000000;
margin-top: 15px;
}

HTML

<div class=”spacerline”>

Webpage is https://www.elitedentalstaffing.com