Re-add Cantarell and Noto Sans for darkly/darkly-red.

[IngrownMink4]

Read more about this change here: #779
I have also added Roboto Mono for Android users.

[jsit]

Read more about this change here: #779 I have also added Roboto Mono for Android users.

1. These CSS files are generated; you should be updating _variables.darkly.scss instead.
2. Why only darkly and not litely?
3. What about Linux users who aren't using GNOME? Ubuntu's system font is Ubuntu.
4. This should probably just be done with system-ui, not a specific font family name: https://css-tricks.com/snippets/css/system-font-stack/

[IngrownMink4]

1. Okay, I'll give it a try.
2. I was thinking of doing it in two different PRs in case there was any particular regression with the combination of colors + typography. But there would be no problem in making the necessary changes in a single PR on all affected topics.
3. Not a bad idea, especially considering the popularity of Ubuntu and Linux Mint. There would be no problem with this (as long as it doesn't replace Cantarell in GNOME).
4. I don't think it's a good idea. Take a look at this article: https://infinnie.github.io/blog/2017/systemui.html