Sidebar doesn't fill the available space

[laurelfulford]

Describe the bug
If you have less content in the sidebar, it won't fill the available space on smaller screens.

To Reproduce
Steps to reproduce the behavior:

1. Add one sidebar widget -- a good one is the search widget, but anything narrow -- say a Navigation widget with a menu full of short links -- will also work.
2. Make the screen narrow enough for the sidebar to stack underneath the primary content area.
3. Scroll down and view the sidebar.
4. Note it doesn't fill the available space.

Screenshots
If applicable, add screenshots to help explain your problem.

Expected behavior
100% wide elements -- the search, the widget headers -- should fill the available space when the sidebar is stacked on smaller screens.