Adding own CSS to Control Panel

I am trying to add a small amount of CSS to the Control Panel to style a data table within the Module I am writing, and that will appear under the Control Panel.

Tried using the

$DSP->extra_css = “path/to/file.css”;

which is mentioned on the Dev page for the Display class. Does not work (my CSS file is not referenced anywhere in the CP webpage source) - any ideas? Has this ‘undocumented’ feature mentioned on the pages comments been removed? or is it wrong or not available?

I am trying to set this - just where I set the title, crumbs, etc

$DSP->title = $LANG->line('fbtcommerce_module_name'); 
        
        $DSP->crumb = $DSP->anchor(BASE. 
                                   AMP.'C=modules'. 
                                   AMP.'M=fbtcommerce', 
                                   $LANG->line('fbtcommerce_module_name')); 
                                    
        $DSP->crumb .= $DSP->crumb_item($LANG->line('view_basket')); 
        
        $DSP->extra_css = "/css/ecommercecp.css";