Styling the drop down sub menus, hover colour

This is something I have been wanting to do for ages, I can';t find any good guides and I have spent the weekend messing with the CSS to no good end result. The best way to describe what I want is to have a drop down menu look just like the drop down on the WP admin site bar.

I have no idea how to achieve this, the CSS menu section is so confusing I just don't know where to start.

Here is an example of how I want the menu to look, slight transparency with same colors, border and shadow.


Here is a link to my site : http://lakewooditsupport.co.uk/