I just created a bookmarklet! Try dragging Edit Styles to your bookmark bar.
This only seems to work in Chrome, Safari, and IE9. If anyone can tell me why appending an element to the DOM in Firefox or Opera creates a blank document, I’d love to know.
You can type any sort of CSS in the box. If there was already some CSS in a <style> element in the head, you can edit that.
Try something like the following: