Sure thing. In the HtmlRenderer properties under BaseStylesheet
<style type="text/css">
*{
</style>
Or
<style type="text/css">
html, body{
</style>
<style type="text/css">
*{
font-size: 20px;</style>
Or
<style type="text/css">
html, body{
font-size: 20px;</style>