OK,
so the default tile is in the index.php, here:
var map = L.map('map').setView([67, -100], 3);
L.tileLayer('http://stamen-tiles-{s}.a.ssl.fastly.net/terrain/{z}/{x}/{y}.{ext}', {
subdomains: 'abcd',
minZoom: 0,
maxZoom: 18,
ext: 'png',
attribution: 'Map tiles by <a href="http://stamen.com">Stamen Design</a>, <a href="http://creativecommons.org/licenses/by/3.0">CC BY 3.0</a> — Map data © <a href="http://www.openstreetmap.org/copyright">OpenStreetMap</a>'
}).addTo(map);
If you scroll below that you will see all the other tiles so you just swapt the URL and the parameters below and choose the one you like