Hello Sean,

the link to online documentation is available here:

http://clapat-themes.com/helpcenter/humpton/

but to change the font size and line height for hero title you can use the following css

.hero-title {
    font-size: 10vw;
    line-height: 13vw;

}

this has viewport units but you can specify the size directly in pixels and then add the code to customizer -> additional CSS

you can use also instructions for different screen widths

@media only screen and (max-width: 767px) {
    .hero-title {
        font-size: 13vw;
        line-height: 17vw;
    }
}