There are very many themes you can choose from. 2 are included with the default install. You can download as many themes as you want and upload them to your server. Switching themes is just one click in the WP admin area. There are quite a few plugins that allow you to embed YouTube videos. Both Themes and Plugins can be found when you scroll to the top of this page and hit the "Extended" link.
As for (X)HTML, it can be added, not only within the post or Page but you can also edit the theme template files with an editor and add as much (X)HTML as you like (although I strenuously recommend you don't use MS Word as your (X)HTML editor like you have been doing for your current site.) There are also counter plugins to make it easy for you.
If you have any more questions or need help setting it up, just ask.