This article is a legacy article. It applies to previous versions of the Liferay product. While the article is no longer maintained, the information may still be applicable.
When deploying themes to Liferay, theme references are created and saved into the database. If the theme is improperly undeployed or, if the theme is not ported to the new Liferay portal post-upgrade, these references will remain within the database generating several WARN messages and possibly ERROR messages. Users will then be unable to access the Liferay portal.
Resolution
The following options are available to access the portal:
1.) Update and Deploy the missing theme to the new Liferay Portal.
or
2.) Add the following properties to the portal-ext.properties file:
default.regular.theme.id=classiccontrol.panel.layout.regular.theme.id=controlpanelor
3.) Navigate to the control panel directly via: http://localhost.com:8080/group/control_panel/ where 'localhost.com:8080' is the location of the Liferay portal. This link should be available if the control panel theme has not been modified. From there navigate to the Portal Settings -> Display Settings and change values for the displayed themes to default (classic).