Changing the ‘active page’ menu item colour

Hi There
Firstly great theme, just finishing off styling etc and I’ve would like to change the colour of the ‘active page’ in the navigation to distinguish which page of the site the user is on. Is this something fairly straight forward to do as my client is keen on the change? Any help would be greatly appreciated.
Many Thanks

For example:
http://john-ellis.co.uk/dev/?page_id=9
I would like “what I do” to be light cyan to indicate the page being viewed.