Hi Maeve,
You can change the color code in the following CSS and then add it in “Appearance => Theme Options => Custom CSS” box. 
.hentry, .widget, .no-results, #author-info, #disqus_thread, #content .error404 {
    border-color: #ddd;
}
If you want to remove then the css will be as below:
.hentry, .widget, .no-results, #author-info, #disqus_thread, #content .error404 {
    border: none;
}
Regards,
Sakin