Hello,
1) This is not our theme issue because I have enabled default Storefront theme and can see the same problem http://prntscr.com/e3mkdy Contact with plugin author to get support.
2) This is not our theme styles. Very likely you are using some additional plugin that adds additional styles for the price filter http://prntscr.com/e3mj53 Add the following custom to remove border
.price_slider_wrapper .ui-slider {
border: none;
}
3) It’s because of your styles in child theme style.css http://prntscr.com/e3mh4l
Regards