Some simple improvements suggestions:

1) Allow users to type in their own text to preview (or at least try and grab the text used within the element we are styling from)

2) Show what the textual CSS styling result will be (on a seperate tab?)

3) Add a splitpane to allow adjustment of the rendered preview

4) If the quick edit is intended to be what most people would like to do most often then it should be the first tab or at least the tab it opens up on by default