The input element wasn't rendered anymore ("Flattening unsafe node"), so build
a document fragment instead of constructing the string for the prompt element.
Instead of adding inline style, we put CSS classes on the elements
for which we add rules in mozilla.css.
The styles are slightly tweaked so we use the colors from the DevTools theme,
and show proper focus indicator.
Differential Revision: https://phabricator.services.mozilla.com/D242525