Mobile Device Menu Colour

Hi I have set the responsive to ON for my website however when I view the website on my mobile i cannot see the menu icon. My background to navigation is set to black. How do i change this so that I can see this so when people visit my website on mobile phone devices they can see that there is a menu icon?

The website is http://www.theperformancestudio.com.au.tmp.anchor.net.au/

PS: Great theme!!

Yvette