Well, Design Studio uses UI5 for component creation and positioning so there are a number of CSS classes that may influence component classes. Below example has a pretty good effect on the overall fonts though (obviously pick an appropriate font and color, these are just for illustration):
In your style.css file:
.sapUiBody,
.sapUiTv,
.sapUiLayoutAbs {
font-family : Wingdings !important;
color : #FF0000 !important;
}
Basically you'll need to derive all the css classes at play (IE F12 tools will help) but my example covers Text View, Body text, and a few others.