Yes, in Template Settings Line-Height is based on "px" as I remember.
In some templates, some header are defined by template styles. You can create custom.cssfile with your own styles that will be used in whole site, it's very simple process.
Example syntax:
h1, h1.sppb-addon-title {font-size: 2.4rem; font-weight: 900;}
h2 {font-size: 2rem; font-weight: 600;}
h3 {font-size: 1.3rem;}
h4 {font-size: 1.2rem;}
@media screen and (max-width: 760px) {
h1 {font-size: 2rem; }
h2 {font-size: 1.8rem; }
}
I hope you read at least one book about CSS so you know the basics.
Remember that if you used Font settings inside SPPB addon it will be "stronger" and it will override template styles.