The theme is responsive. However, you’ve set the width property on several elements so the site isn’t able to resize properly. For example, this rule:
.site-content .entry-content, .site-content .entry-header, .site-content .entry-summary {
color: #555;
width: 1080px;
}
and this rule:
#homeLinks {
display: block;
margin: 0 auto;
width: 1080px;
}
When you set the width to 1080px for those elements, then the width of the page stays at 1080px, which is why on a mobile device, like a cell phone, your users are going to have to scroll to the right to see the content.
You should remove those width properties and your site will respond much better.