custom.css

<style type="text/css">
@import url('http://fonts.googleapis.com/css?family=Source+Code+Pro');

/* Change code font */
.CodeMirror pre {
    font-family: 'Source Code Pro', Consolas, monocco, monospace;
}

div.text_cell {  
    max-width: 105ex; /* instead of 100%, */
}

div.text_cell_render h1 {  
    font-size: 16pt;
    font-weight: bold;
    color: RoyalBlue;
}

div.text_cell_render h2 {  
    font-size: 14pt;
    font-weight: bold;
    font-style: italic;
    color: RoyalBlue;
}

div.text_cell_render h3 {  
    font-size: 14pt;
    font-weight: normal;
    font-style: italic;
    color: RoyalBlue;
}

div.text_cell_render .rendered_html pre,
div.text_cell_render .rendered_html code {
    font-size: 14pt;
    color: RoyalBlue;
}

.rendered_html ol {
    list-style:decimal;
    margin: 1em 2em;
}

hr {
  height:0; 
  border:0; 
  border-top:5px solid RoyalBlue; 
}
</style>