If you've read both analysis of .html files and tried to follow the quick "How to", you should now have an idea of how to customize your own.
As I said earlier, when you mess in files, always be sure to keep a backup copy (in case) to which you can revert quickly.
I hope this tutorial has nicely introduced you to HTML5 and CSS3 and gave you ideas of how you can really improve the display of your games with a "few" lines.
As for anything, mastering come from practice (and I don't mean that I'm a master, just an enlightened amateur), so the more pages you'll make, the easier it will be for you to make those.
To sum up a little:
+ Styling is defined in CSS
+ Style is applied in HTML
+ The parameter 'id="name"' is an important element that stands as anchor and styling selector. The 'name' should be unique for all the page.
+ The parameter 'class="name"' can be applied several times in the page.
+ <div> is the base tag for your custom styles but be sure to check HTML existing tags or even this HTML5 cheat sheet to see the existing and useful tags in HTML5. Using the proper tags will make sure your page can be referenced by search engines.
+ Any open HTML tag opened must be closed.
+ Any CSS property defined should be ended with a ";"
Have fun customizing your own exported projects.