I was also wondering how to center the Page titles.
In your custom CSS, you currently have this:
#h1{text-align:center}
That won't work because h1 is an HTML element and it doesn't need a "#" in front of it for CSS. In CSS, you can use the "#" to target HTML elements that have ID values. For example, if you had HTML that said <h1 id="hello">Title</h1> then you could target it by using "#hello" in your CSS to target only that specific title and not any others or just "h1" to target all h1 elements throughout the site.
To center the titles on pages in the Twenty Eleven theme, you can use a class selector. It's similar to an ID selector, except you use a "." instead of a "#" to target. Here is an example:
.singular .entry-title {
text-align: center;
}
The ".singlular" part means that it just affects single pages and not posts or other types of pages.