Theming and designing css in Drupal 7
Well that's  very vast topic, however few things I would like to share with you are:

The best place to start can be the documentation of ZEN theme: https://drupal.org/node/193318. They are having pretty good maintained doc.
Must go through tutorial is drupal's theming guide: https://drupal.org/documentation/theme
To undertand the basics of theming: http://drupalize.me/videos/introduction-theming-basics-drupal-7
How to create the subtheme: here is official doc on how to create a sub-theme as per drupal standard.
Also there is one nice video tutorial on creating a sub-theme of Zen: http://www.youtube.com/watch?v=y-IXpbdgA3A
One good answer on creating a theme with drush command: How do I create a Zen&#39;s sub-theme using Drush?
Moving further you can check how do drupal process theme variables for templates: Setting up variables for use in a template (preprocess and process functions)
It would be good to learn few of the CSS frameworks: SASS framework, LESS framework

I tried to cover few of the quick points that striked me first, but anyways there is no limit of learning, so keep on exploring!!