Themes from plugins is still referenced by Confluence after plugin is uninstalled

Confluence still uses an uninstalled theme's layout (eg. the font will still be the old plugin's font) as its site layout although the plugin has been uninstalled.

Steps to replicate:

1. Install a theme plugin
2. Set the theme as the Site Theme
3. Uninstall the theme
4. Confluence theme selection page (http://ConfluenceURL/admin/choosetheme.action) will report that the site is using the default theme. Warnings will be populating the log file:

   2009-03-10 16:11:26,659 WARN [http-8080-3] [atlassian.confluence.themes.DefaultThemeManager] extractTheme Unable to find configured theme module: com.atlassian.confluence.themes.classic:classic
    -- referer: http://localhost:8080/admin/console.action | url: /admin/choosetheme.action | userName: admin
   

Customers who used the Classic Theme as a site layout might encounter this warning after they have upgraded to Confluence 2.10.

The work around is to set Confluence site theme to other installed theme.