Style the DevTools panel in a way that matches the browsers' DevTools appearance more closely, and makes more use of the available screen space.
Support light and dark DevTools themes, including theme changes on-the-fly in Firefox.
Streamline the base gui.html a bit due to the simplified styles, and demarcate the DevTools and non-DevTools parts of the UI in this file completely, rather than having to remove elements in JS after load.
Streamline the build script a bit in light of the above.
Also rename the "tag" used to demarcate stuff to do with the sidebar in gui.html and options.html from "ui" to "sidebar" for clarity.
Add a bit of info abut the mutation stats at the bottom of the DevTools panel.
Re-order the buttons/warnings disclosure in the DevTools panel to be more consistent.
Style the buttons/warnings disclosure to look more interactive.
Fix some layout issues in Firefox (using feature detection—thanks StackOverflow—this could be used for other styling too).
Fixes #405