I've made a copy of the Default theme CSS (dark.css) in order to modify it. Basically I need to set the default font larger and whiter colour for better visibility. What "html" page do I attach it to for testing or should I leave the copy as the original (darkORG.css) and use my modified copy as dark.css so all calls to it work?

As long as you keep the css file in the styles folder you can name it whatever you want and that name will show up in the admin under options--->theme options