attempt to fix sidebar z-index

I'm not sure that this PR won't have knock-on effects on some installs, so it's intended first and foremost for testing. It is an attempt to solve the issue raised in https://github.com/cuny-academic-commons/cbox-theme/issues/210 and reposted in https://github.com/cuny-academic-commons/cbox-theme/issues/217

Works for me - feedback welcomed.