I am trying to edit the css.php file so that my Pages widget (displayed on left sidebar) shows all parent items and only the sub items of current page.
Have also looked at editing the header.php but have yet to achieve the desired effect

I would suggest you use CSS Inserts instead. This way you can change releases without having to go back in and change teh code again.

If you a trying to change the way the pages widget works, why not look at some of the other page widgets like 'Flexi pages widget'?