You could add the following to ATO>Add HTML/CSS Inserts>CSS Inserts and tweak the settings.
HTML Code:
h1 { font-size: 34px; line-height: 1.2; margin: 0.3em 0 10px; }
h2 { font-size: 28px; line-height: 1.3; margin: 1em 0 .2em; }
h3 { font-size: 24px; line-height: 1.3; margin: 1em 0 .2em; }
h4 { font-size: 19px; margin: 1.33em 0 .2em; }
h5 { font-size: 1.3em; margin: 1.67em 0; font-weight: bold; }
h6 { font-size: 1.15em; margin: 1.67em 0; font-weight: bold; }
These are part of the style for the "Classic" style that comes with Atahualpa. There are several styles that come with Atahualpa for various starting sytles. They are are in the styles folder if you download and unzip the theme file. They can be imported in ATO>Export/Import settings. Just remember to export your current setting for a backup before importing any new setting as the import will override all the settings you have.