Dynamic Stylesheet not picked up

I’ve created a dynamic stylesheet and called it style.php

I’ve put it in my themes/theme_name directory and deleted the style.css that was in there.

Now it looks like it does not pick up the stylesheet at all.

Where do I change the name of the stylesheet to style.php and not style.css? I think wordpress is looking for style.css and now can’t find it.

Thanks in Advance