You don't need special software and as long as the WordPress theme author doesn't disallow it (I have yet to see one who has), you can most certainly upload a pre-existing WordPress theme and work directly from it, tweaking and adjusting to fit your needs.
Where a lot of WordPress themes do use a lot of images for their backgrounds, headers, footers, and whatnot, a LOT of the effects are all CSS code-based, so to really handle editing a theme well, it definitely helps to understand and be able to code CSS.
For various code-related reasons, theming WordPress is more difficult than theming something like MySpace. What makes WordPress do what it does and look how it looks is, for the most part, code. Lots and lots of code.
With that said, unless you have prior front-end web coding abilities and a good understanding of PHP of WP-specific coding, it is easier to use someone else's theme and work from it/tweak it and learn from it than to just dive straight into theming your own theme from scratch.
My recommendation is to find a good theme that you feel suits what you want best - general layout, maybe even colors - and then tweak it to suit.
Good luck!