I use
Top Style for CSS editing -it is a WYSIWYG editor and the coolest thing about it is you can use an html document for the preview window. Just go to your forum, view source, save the html as a file on your PC and you have the perfect web page to preview your CSS changes
Hint: Make a seperate html file for the main page, post index, show flat, etc. and then change the preview document to each one to see how your CSS changes effect all the pages in Threads...
