Remove "dark mode" for posts view.

Just make a dark theme.
Formatting.
Clearer CSS filenames.
Handle posts view stylesheets using themes.
Re-add dark default CSS theme as separate CSS file.
Don't set default theme as its own parent.
Can specify a pattern image for a color in a theme.
Give each theme a description and a color.
Forum-specific themes can be loaded.
Formatting.
Theme equality and -description.
Singleton theme loader.

Don't have to use it, but it's there.
Page settings view takes and uses themes.
Implement UINavigationController subclasses.

One for better theming support. The other for using Awful navbars/toolbars and
state restoration.
xcodeproj ordering.
Genericize forum-specific theme settings.

For now, this includes hacky loading view support. Should theme that properly
at some point!
Copyedit.
Hide pointlessly-exposed function.