Hi Kerry,
No worries
I'll try and explain it a bit more.
1) Yes you need to create a new stylesheet, and you're right when you go to edit stylesheet it only shows your own stylesheets not the defaults. Don't worry about that for now, just create a new stylesheet.
2) Since you're using Firefox, the web developer plugin is by far the easiest way to do this. Read about the web developer plugin here:
http://pbasewiki.srijith.net/howto/playing_with_css Now open 2 different Firefox windows, in one of them you edit your new stylesheet. In the other you open one of your own galleries and use the web developer tool to edit the css. This way you'll be editing the current css meaning all the classes from the Woodmist stylesheet will be there.
3) When you're happy with your edits, you need to copy the whole stylesheet and save it instead of your own. Activate the stylesheet in web developer you've been editing, select all text and copy it.
4) Now switch to the window you opened with the edit page for your own css and delete all the text - and paste the whole stylesheet you just copied from the webdeveloper window. Remember to save!
5) Now just edit your gallery and select your own stylesheet and then update gallery. Now the gallery should be showing with your stylesheet!
Note: Open different windows in Firefox, not just different tabs. Web developer works across tabs so if you switch tabs you'll really make a mess of things
Hope this helps you get started, if not let me know.
all the best,
Flemming