Hi, as part of #1238484: Ability to mark the form buttons to be of a specific type (so that they can be styled differently), the buttons.png file was removed, but .ui-slider .ui-slider-handle and .ui-dialog .ui-dialog-buttonpane button still reference it in jquery.ui.theme.css
The effect is that when trying to place a jQuery UI slider in Seven, a GET http://mysite/core/themes/seven/images/buttons.png 404 (Not Found) error message is thrown in the browser console, and the slider handle retains a transparent background.