Cannot change font for "Sign in" Box

Cannot change font for "Sign in" Box

I have set the font for everything to Arial but I cannot seem to make it work that the "Sign in" Box shows anything than times new roman. Which file do I have to alter to set Arial?
With the developer tools of the browser I see that there is:

<style>
 .fntFamily{font-family: times new roman,times,serif;}
 .fntSize{font-size:12px;}
 .common-textcolor{color:#e2001a !important;}
 .common-bgcolor{background:#e2001a !important;}
 .common-bordercolor{border-color:#e2001a !important;}
 .adsfntFamily{font-family: times new roman,times,serif;}
 .adsfntSize{font-size:12px;}
</style>

but which file is it?

I want to go live with this product, but my marketing department will go nuts with this..