Your theme is calling a google hosted jQuery UI CSS file, which includes the references to the images for that calendar. However, the image references are relative URLs. What you can do is just add the following custom CSS to make those image references have absolute paths. For example:
.ui-icon { width: 16px; height: 16px; background-image: url(https://ajax.googleapis.com/ajax/libs/jqueryui/1.8.21/themes/base/images/ui-icons_222222_256x240.png)/*{iconsContent}*/; }
.ui-widget-content .ui-icon {background-image: url(https://ajax.googleapis.com/ajax/libs/jqueryui/1.8.21/themes/base/images/ui-icons_222222_256x240.png)/*{iconsContent}*/; }
.ui-widget-header .ui-icon {background-image: url(https://ajax.googleapis.com/ajax/libs/jqueryui/1.8.21/themes/base/images/ui-icons_222222_256x240.png)/*{iconsHeader}*/; }
.ui-state-default .ui-icon { background-image: url(https://ajax.googleapis.com/ajax/libs/jqueryui/1.8.21/themes/base/images/ui-icons_888888_256x240.png)/*{iconsDefault}*/; }
.ui-state-hover .ui-icon, .ui-state-focus .ui-icon {background-image: url(https://ajax.googleapis.com/ajax/libs/jqueryui/1.8.21/themes/base/images/ui-icons_454545_256x240.png)/*{iconsHover}*/; }
.ui-state-active .ui-icon {background-image: url(https://ajax.googleapis.com/ajax/libs/jqueryui/1.8.21/themes/base/images/ui-icons_454545_256x240.png)/*{iconsActive}*/; }
.ui-state-highlight .ui-icon {background-image: url(https://ajax.googleapis.com/ajax/libs/jqueryui/1.8.21/themes/base/images/ui-icons_2e83ff_256x240.png)/*{iconsHighlight}*/; }
.ui-state-error .ui-icon, .ui-state-error-text .ui-icon {background-image: url(https://ajax.googleapis.com/ajax/libs/jqueryui/1.8.21/themes/base/images/ui-icons_cd0a0a_256x240.png)/*{iconsError}*/; }