I did end up finding the answer, thanks to the Wiki here. In case anyone else is wanting to do this, what worked for me was to create a file called userstyle.css and load it into the /conf folder.
This is what the file looks like:
/**
* This file provides styles for headings - Suzy
*/
h1 {
font-size:2em;
font-weight:700;
}
h2 {
font-size:1.6em;
font-weight:700;
}
h3 {
font-size:1.2em;
font-weight:700;
}
h4 {
font-size:1em;
font-weight:700;
}