Hi Darren,
It is not clear from your post what you actually want to do.
If you want to create your own visual theme for Moodle, then you might consider studying the Themes documentation. Basically you don't create a brand-new theme from scratch, you copy an existing theme (e.g. standard) and then you customize it. It is almost impossible to provide a complete list of all the elements (CSS classes) in a Moodle site, but you can get good results by using these techniques: a) browsing through existing themes CSS files b) using the Chameleon theme as suggested by A.T. Wyatt c) installing the excellent Webdeveloper plugin in FireFox.
"Is there somewhere I can edit to create just one css sheet for the whole site" -> you don't create a CSS sheet for a whole Moodle site, but you may do so for a Moodle theme. Maybe you would like to have a unique CSS file in your own personal theme rather than separate styles_color.css, styles_fonts.css, styles_layout.css etc. files. This is what I have done in my own orangechoc theme. On the topic of single vs multiple CSS files, see this discussion.
I do not understand "it seems to me that the Frontpage is a combination of the standard and the standardwhite themes." The way the front page of a Moodle site looks like depends on the chosen theme for that site. If you select standardwhite as your site default theme, then indeed standardwhite is based on standard. In fact most Moodle Themes are based on the standard theme. Again, all of this is well-explained in the Themes Documentation and can also be understood when looking at all the files in any given theme distributed with Moodle.
Finally, some themes will work only with some versions of Moodle, so it is important to know which Moodle version you are using.
Hope that helps,
Joseph