Parse CSS Regex accepts invalid code

Hi,

In acp_styles.php the RegEx in the db_theme_data() function;

@import url(["\'](.*)["\']);

will match invalid code, e.g. @import url("common.css'); as well as empty declarations. An alternative would be;

@import\s+url((["\'])(.+)\1);

Obviously the 1 needs changing to a 2 in the $matches[1][$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);
@import url("common.css");
*/