Border

.wp-show-posts-inner {
    background: #FFFFFF;
    border: 1px solid #000000;
}

Where to put this cod?
In CSS?

You can use a plugin like this: https://www.ads-software.com/plugins/simple-css/

Or the Additional CSS section in the Customizer in WP 4.7.

This is also a feature in the pro version of WPSP.