You don't need a special tool to do this. All of the skinning work we do at i7MEDIA is done on live demo sites so that we can test from multiple computers and browsers. You can take the same approach. All you need to do is disable caching on the site and when you make a change to your CSS or layout.master, upload it to the site. We use Aptana (with a bunch of file-type configuration to support the .net file-types) or Notepad++ with the FTP extension. The only real reasons we use either of these tools are: 1) FTP Support, 2) Free, 3) Fast and low resource utilization. Expression Web and DreamWeaver have neither of the last two.
Also, you can use the Design Tools located under Advanced Tools in the Administration Menu to modify your CSS. The changes will be immediate (if caching has been disabled). You can not however modify the layout.master.
We use this approach with our customers and they like it a lot because we can change things like font folors and types and they see the results immediately.
Remember when you are finished to enable caching.