I recommend using Firebug to see what code overrides what. The best way to edit css is to create a child theme and override the styles from there. You can check out the child theme examples how this is done.
All styles set in style.css will be overridden without any more specificity, but the styles set in the theme settings will need some extra specificity. Example how to override stuff set in the body tag in the settings script (adding html before the body overrides it):
html body { font-size: 15px; background-color: #eee; }
btw, you should change the H1 size in the theme settings ("Article header & H1-tag size" in the typography tab), and that is the best way, since the post headings vary between H1 and H2 (SEO improvement). and, there is no point to edit optionstyles.css since this file is regenerated every time you save the settings (if the "Cache the settings stylesheet" setting is set).