data: readability.css: Use sans-serif as fallback for dark mode.

We *don't* want to use a monospaced font like that for console mode.

Signed-off-by: Rogério Brito <rbrito@ime.usp.br>
data: readability.css: Allow use of DejaVu Sans for dark mode.

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