I thought it would be easy to create an extension that lets you adjust CSS properties using JQuery UI sliders. About three thousand and something lines of code later, I have made https://github.com/pdorrell/style-adjuster, which is not even finished yet, but it demonstrably functions.

In the course of development, I found that it is non-trivial to make a popup window inside a Chrome extension which acts on a target tab, because "window.opener" is not available. So I developed a test extension at https://github.com/pdorrell/test-popup-extension which does that, and which runs its Javascript code interchangeably inside or outside the extension.