On the filters page on StackExchange.com, a static external image is loaded to force a background colour instead of applying a CSS
background-color for the HTML element
.filters-sidebar .filters ul li.current. Here's the image:
My recommendation is to apply a CSS
background-color to the element instead of loading a static background image from a server. To generate the desired shape, you can also use CSS.
In plain English, the area affected is the name of the selected filter in the "Select Filter" box near the top of the page, e.g. "Favorite Tags":