lib/main.js: Avoid deprecated use of `require`.

Otherwise, we get a lot of warnings when using `cfx xpi`.

Signed-off-by: Rogério Brito <rbrito@ime.usp.br>
data: readability.css: Put space for dark theme in the floating bar.

It is still not that great, but better than the previous situation.  We
should probably use dimensinons that are not fixed, but dependent on the
size of the window.

Signed-off-by: Rogério Brito <rbrito@ime.usp.br>