One thing you cannot do on a Kobo is use font-family in the body style. It doesn't work. Kobo ignores it. You have to use font-family in other CSS styles such as p.
Code:
p {
font-family: Charis
margin-top: 0;
margin-bottom: 0;
text-indent: 1.2em
}
All p will be in the font you've chosen. This works. But in the body, it does not work. This is a long standing bug that Kobo feels it's OK to ignore when really, it's not.