Have a Drupal site, have a Photoshop design, and want it converted into a responsive theme? I do that. I also edit existing Drupal themes, and make the exsting non-responsive themes responsive. Responsive means that the Drupal themes will adjust themselves for mobile devices and narrower screen resolutions to look good whatever the width.
My own rule of thumb is that every theme that I create is responsive. Thus, if you want me to theme for you, you will get a responsive theme as a result. Sometimes designers design their themes in Photoshop or Fireworks for wide screens or very wide screens, without really thinking what will happen for the narrowers screens. If I see design patterns that won't scale down well, I will warn you about it. In some cases reworking or simplification will solve the issue, or agreeing on how the elements will stack.
For theming, I prefer a starter theme with built-in grid. By default, I use Twitter Bootstrap as a starter theme, which takes care of many a design pattern and grid layout of the box. However, I can use other grid systems and starter themes, like Omega, Zen, AdaptiveTheme, Zurb Foundation and Skeleton Theme if client so requests.
I don't consider design as my primary trade, but I will also wireframe and design a clean content-centerd Drupal theme for you if needed. See an example "Pudding" theme.
In my work, I use HTML5 and CSS3, CSS with the SCSS preprocessor for maintainability. You can request theming to be done in LESS or pure CSS without a preprocessor if there is a specific need for that.