There are often multiple css files, referred to by the theme depending on the page. Also, a module may have it's own css files - there are often 10 to 20 css files used for a single webpage.
Design Companies and Drupal Themes
It is difficult to find someone who can design a theme and administer a drupal website, so you may need to hire a professional organization and divide the work: a graphic designer can re-design the website style, then a drupal programmer can convert this into a drupal theme.
Most designers have not worked with the Drupal software and think they need to do so. Thus they will charge you for the time it takes to learn how to install and run drupal. If you want to stay within budget, plan your theme development in 2 stages.
Drupal themes are built with css files. This is not a drupal standard, rather an Internet standard. Designers create the style sheets (css files) for themes. Your drupal website probably already has a functional, multi-browser compliant theme, so all that is required is a re-designed style sheet (and color scheme, graphics, header....?)
Drupal themes can be re-designed independent of the drupal platform. In fact, from your web browser you can see all the elements to be styled.
When re-designing a theme, it is best to add a new style sheet. In the past, designers would edit the elements in the existing file: (ie. style.css, the main style sheet used in a many themes). The problem with this is that the original theme cannot be updated. An update would write over the changes that were made to the theme's style sheets.
Designing Style Sheets
A designer does not need to be running drupal to make changes, rather they can use Firefox tools like Firebug to re-design style sheets on their computer. A designer then emails the new style.css style sheet to me and I can activate live on the website in seconds!
A designer creates style sheets or just a wire-frame design. The wire-frame can be a Photoshop or just a graphic file. I can then translate this into css and a Drupal theme.
The most cost-effective, easiest solution is for someone there to re-design the website style, then a drupal programmer can convert this into a drupal theme.