in style.css of the theme.
if you go through the file line by line, you should be able to find the styles that define the h1 to h6.
(if they don't exist, you can add them at any time.)
be aware that some of the titles in the theme are linked, so that the actual visible style might be the one of the html link tag; for instance h1 a {...}
some of the styles might also be more specific such as
#content h1,
#content h2,
#content h3,
#content h4,
#content h5,
#content h6 {
color: #000;
margin: 0 0 20px 0;
line-height: 1.5em;
}
or
h3#comments-title {
padding: 24px 0;
}
or
h3#reply-title {
margin: 18px 0;
}