I have built a new SharePoint 2013 environment and after provisioning a new Search Service Application the following was regularly being logged in the Application Event Log:

Content Plugin cannot be initialized –  list of CSS addresses is not set

eventid 2548 css address not set sharepoint search

After a fair amount of Googling and looking through logs I learned that this was caused by the Search Service Application not being fully provisioned.  After removing and recreating the Search Service Application these errors went away.