theming

I recently posted a question over at Drupal Answers - Stack Overflow which I thought might be better addressed here.

I am simply trying to change the "more-link" on a node display. My understanding was that that could be done by
adding an implementation of theme_more_link to template.php. However, that doesn't appear to work. I then noticed the
HOOK_theme implementation with its comment: // @TODO: Needs detailed comments. Patches welcome!

I tried adding a line:

Hi there, i was wondering if you can help me, i have recently installed the latest Zen theme to start work on a new site and noticed it's changed slightly with the amount of stylesheets involved now. This is fine but i need to know what you recommend to do with the sub-themes. It's giving me a headache battling with so many stylesheets and switching from one to another to do some simple styling.

What's the best practice, should i be working on my own .css file overwriting these stylesheets or just amend each one as i go along?

Cheers for your help!

Rik