Glad to hear it worked.
You can still use <center> and it still works. (It is deprecated in HTML5, the upcoming standard, but it still works). For your own sites, you can use it, it works. The main reason is the modern web encourages separating content and styling. That is, <center> tells text HOW to display, just like the old <font> tag. In modern terms that is what the stylesheet is for. HTML specifies WHAT to display, CSS, how it should look,
On your own sites it doesn't matter that much. But if you are going to learn to do these things, you might as well learn the ways currently considered best. That too will change over time, of course.
I tried giving you code that would work with any Contact Form 7 form so you didn't have to look up the form ID. But it was getting overwritten by CSS the browser assigned a higher priority. That would require even more CSS to override the overriding CSS. So I felt using the form ID once per form was going to be the simplest way for you.
A little more about centering:
CSS has two classes of tags, block and inline.
<p>, <div>, <form>, <h2> and the like are considered block tags. They CONTAIN other tags and elements. Its with Block tags you must use margin: 0 auto to center the text, after assigning it a width.
The block tags contain other content called inline - straight text, tags like
<strong>, <img>, <span>. When you assign text-align: center or wrap the
<center> tags, the INTERNAL content of the block containing tag is centered WITHIN the block tag.
You can't center
<form> with that because they are block tags to begin with. They are centered as I explained before, set a width and use margin: 0 auto.
Once you understand the principle it's much easier to know how to center things.