When you change the size of the screen to phone/tablet it is responsive and has different menu bar, I cant find in any of the css files in my themes sub folder the colour of the hover or buttons it self.
image-displays.com is the site
the path to your css file is http://image-displays.com/skin/frontend/default/theme541/css/superfish.css
To find this out, most comfortable way is to use a browser plugin like firebug for Firefox. For most browser there something similar available. You can even change the css setting live in Browser and see the behaviour without being persistent.
Take a look at the screenshoot. I changed the color of the "CATEGORIES" to yellow. As you can see in Firebug the name of the selector is #menu-icon and its on line 170, the right attribute is color. In the next screenshoot I marked you the '+' button its an image, its resides just 2 lines above the color in the same selector like the text.
superfish.css
@ line 199.