If you're going to customize the theme then you should learn both PHP and CSS. But I think for your site to look like the other one you should concentrate more on css and html. You can find tutorials at w3cshools.

CSS - PHP - HTML. Would that be a suitable order?

Thanks both for your responses. It looks like w3schools is the perfect place for me.

ChrisStCriox, if you're not too busy. Do you mind trying to remember that theme?

What do you want? how fast? what is your learning strategy?

I'd go with:
"All at the same time in small doses"
(like mixed martial arts)

Because it might take a while to learn one first, then the other, then the other...
I'd start with Wordpress per se and fill in the blanks while you're tweaking and learning, specially since you have a project going on right now.
As time goes by you can go deep into just one and then other languages.

Example:
http://blog.doteasy.com/2010/08/18/tweaking-your-wordpress-theme-part-1/

Agreed its best to learn all 3 at the same time, however in the beginning I would concentrate more on understanding CSS and HTML while learning PHP along the way.

Why??

Regardless of what platform you use it will always send the information for the browser to display in an HTML format and the CSS is then used to tell the browser how it wants that information displayed on the page

PHP does cool stuff behind the scenes and then the results of the cool stuff is converted into an HTML format which it the sends to the web browser for it to display in whatever way the CSS tells it to

good luck, you have some fun times ahead

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