When filtering a search, you will add an attribute identifier definition to filter the results, you'll then be able to do it this way:

    {set $search=fetch(ezfind,search,
                       hash(
                       	query,$search_text,                       	,
                       	filter,array( 'article/theme:test' )
                       ))}

In this example, I'm waiting articles for which the attribute "theme" will match the value "test". But the class name isn't kept in the solr request, as the filter is transformed in "attr_theme_t:test", and then if any other class defines an attribute identified "theme" the contents matching the criteria will also show up. If a type "folder" also defines an attribute "theme", then folders will also be part of the results.
Indeed adding a "class_id" criteria can be added to filter articles and then perform the good search, but it should be implicit.