Change the sidebar expand background?

you can change the sidebar background color to another color or background image using the

.main-inner.group class selector found in the base:sidebar section… I changed it to a background image, and it works fine except for when the sidebar collapses at smaller screen sizes. How can you change the same background in that situation… I tried adding a bg image to .s1-expand .s1 & .s2-expand .s2 but it did not change the bg image of the sidebar content area.