If you're not using a Child Theme, and if the theme doesn't have a designated section of the dashboard for CSS modifications then install this Custom CSS Manager plugin http://wordpress.org/plugins/custom-css-manager-plugin
Then use its "CSS Code" section of the dashboard to hold your CSS modifications:
Alternatively use your Child Theme style.css file to hold your CSS modifications:
#pre-header,
#sub-footer {
background: deeppink;
}