Visual Web Develper Designer ? 

I use MS Expression Web.  This allows the skin folder to be loaded as a "website". 

Typically I drag the css file onto the index.aspx to link it temporarily while I work on the css - usually open in another window. (don't forget to delete this link when you deploy the skin like I often do)

The interface is fantastic and there is a very generous upgrade path. Have you ever owned a copy of front page / dreamweaver and others - even if you lost both the disk and serial years ago - you qualify.

Intellisense does the same useful work - just checked this includes skin object attributes.
You can have multiple instance open at once.

For many VWD is expensive and provides many features that are simply never used.

My other skinning tool tip is ColorPic - a free utility that allows colours to be picked and stored as named palettes. Very useful to hold a sites colours in an easy to recognise way.
http://www.iconico.com/colorpic/

click click

Ian