Ok, I have checked you website and you main file index.html and you have to change the code in the line 87 to this one:
<li><button class="pt-trigger left-margin css3Animate" data-animation="22" data-goto="6" data-style="lightNav" data-style2="blogNav">Contact<span class="line css3Animate"></span></button></li>
Remember: the style is set in the attribute data-style. In your case, you have the option darkNav and you have to change it to lightNav.
And it woul be great if you remove the line 88 with this code:
<li><button class="pt-trigger css3Animate" data-animation="22" data-goto="" data-style="lightNav" data-style2="contactNav"><span class="line css3Animate"></span></button></li>
Regards