In acp_styles.php the RegEx in the db_theme_data() function;
will match invalid code, e.g. @import url("common.css'); as well as empty declarations. An alternative would be;
Obviously the 1 needs changing to a 2 in the $matches[$idx] str_replaces().
I believe it would be a good idea to ad a note to the top of stylesheet.css such as;
Please note that if you require the CSS to be parsed for template variables phpBB only supports the @import notation below (using single or double quotes);