"Stretch the sidebars and the content area down to the footer" does not work.
The script which is responsible for stretching sidebars is called only when "Optimize the website for viewing on mobile devices" option is enabled. This is considered as a software issue #PPB-4330.
This issue has been fixed in Parallels Plesk 12.0.18 MU#29, make sure that corresponding MU is installed. As a workaround enable "Optimize the website for viewing on mobile devices" option.