I see the problem, Aristo theme doesn't remove tooltip's styles. Actually it doesn't provide css style for tooltip, that's why the style is missing.
So you should at least add this to your css file then:
.ui-tooltip {
padding: 8px;
position: absolute;
z-index: 9999;
max-width: 300px;
-webkit-box-shadow: 0 0 5px #aaa;
box-shadow: 0 0 5px #aaa;
}
body .ui-tooltip {
border-width: 2px;
}
/* Copied from jQuery UI css */
Hope this helps ;)