http://scott-enriquez.com/ is indeed setup here at WordPress.com. @castelinokelvin, a quick check of the footer link will usually give you the right info about that.
Regarding CSS not working when you think it should, I would recommend checking it using your browser's web inspector. If you right-click on an element and select "Inspect Element," then you should see a panel open with the HTML for what you clicked on as well as all of the CSS that applies to it. You can usually check or uncheck different pieces to turn them on or off to see what's doing what. It's a really nice way to learn about the CSS of a site.
CSS has all of these specificity rules. The most specific selector wins, or if there is more than one rule using selectors with the same specificity, then whatever is loaded last wins. In your case, just using the selector "h2" is not enough in the Oxygen theme because that theme uses a selector that is more specific: ".entry-title a"
Here is an example you can try out. The top one is for the main site title (or what I would think of as the blog title), and the bottom rule is for post titles. You'll notice there are two because on the home page or archives pages the post titles are links (so you need to add "a" to the selector) and the individual posts just use ".entry-title". I added "#page" to the selectors to make them even more specific so they will override other stuff in the theme:
#page .site-title a {
color: #f00;
}
#page .entry-title,
#page .entry-title a {
color: #0f0;
}