Add new article themes.

[toolstack]

This PR does several things related to article themes:

• Adds a classic article theme that matches the style before PR Make various theme improvements #566.
• Adds a article theme that matches the elementary feed list theme.
  -Updates the displayed text for the "Midnight" them to include a reference to the Dark article theme ("Midnight (Dark)").
• Adds theme titles in the style.css file for those that were missing one.

[bilelmoussaoui]

Would love to merge this but I have started working on #479 and sadly due to some lack of time I wasn't able to finish the work there. I would like to work on it this weekend. Once it's merged, there will be a new whole theme system that should work much better than the current implementation. You will be able to create a proper theme instead of adding everything on the same CSS file.

[brendanlong]

I tried to review this but I can't get the updated schema to work. Selecting a different theme doesn't do anything, and I get this output on the console:

(feedreader:7226): GLib-GIO-CRITICAL **: 18:57:55.490: g_settings_set_enum(): invalid enum value 5 for key 'article-theme' in schema 'org.gnome.feedreader'.  Doing nothing.

(feedreader:7226): GLib-GIO-CRITICAL **: 18:57:57.106: g_settings_set_enum(): invalid enum value 4 for key 'article-theme' in schema 'org.gnome.feedreader'.  Doing nothing.

I'm not sure how to refresh that.

[toolstack]

@bilelmoussaoui I saw #479 but it look stalled, hence this PR. I'd be happy to create separate css files for the new themes once that's ready.

[toolstack]

@brendanlong make sure enums.vala is updated with the new entries.

[bilelmoussaoui]

Will update my PR in the next few days so you can push your themes already on it

[toolstack]

@bilelmoussaoui any update on the theme's PR?

[bilelmoussaoui]

Sorry for the delay guys. Should land tonight/tomorrow morning