You can do it the way the article specifies (edit the style.css file of your theme by going to Appearance > Editor), but I would suggest not editing it. Instead, take a look at what rules are in effect for the element that you want to change by using a web debugging tool like Firebug or Chrome Developer Tools, then add your own CSS rules to override the existing rules using JetPack’s Custom CSS option (I see you have JetPack installed).
For example, using Chrome DevTools, I can see that your blog titles are styled using this rule:
h2, h2 a, h2 a:visited {
color: rgb(251, 85, 99);
font-family: 'Alex Brush';
font-size: 34px;
text-decoration: none;
}
The blog titles also have a class of entry-title, so to modify the styling of your blog titles, you can add this rule to JetPack’s Custom CSS option:
h2.entry-title {
color: rgb(251, 85, 99);
font-family: 'Tahoma';
font-size: 30px;
}
Change the value for font-family to whatever font you want.