CSS works by use of inheritance, so if you define a font globally, and there a different font is defined on an element below this, it may not take. However, quickly you could try the following:
body{ font-family:Arial !important;}
Substitute the font you want for Arial. The !important part tries to enforce the rule so it should be taken above all others. Though, there could be a fixes you’ll need to make, but you should be able to then track where to remove the references.