The theme I'm using mentions how to override the default style. But how do I see the default style sheet that is being used? For one thing, I changed the top headline of the markdown file that comes as the default and it made the title of the page match, which was totally unexpected. But, I'm wondering if this is a Jekyll convention or something and has nothing to do with a stylesheet (I don't even think stylesheets can do that?) The Jekyll documentation is mostly geared toward those wanting to install Ruby and play around locally first. I'm just trying to figure out the basics on Github first before going that route.
You should be able to use the Developer Tools in whatever browser you're using to view your GitHub Pages site to get the full CSS that is being used to display your page. In general, if you want to understand how any web page is being rendered, the Developer Tools is often the first stop.
Let us know if you have more questions.