More themes?

[quank123wip]

mdbook theme is pretty standarized, maybe adding more themes is a cool option. (I'm interested to make one)

[Myriad-Dreamin]

In fact we has supported a theme option in CLI, just to update documentation:

shiroa/cli/src/lib.rs

Lines 98 to 103 in 66bd468

	/// Specify a theme directory to copy recursively.
	///
	/// The files will be copied to the `theme/` in the output
	/// directory.
	#[clap(long)]
	pub theme: Option<String>,

You just start to create your theme with following steps:

1. copy the default theme, themes/mdbook, to other place, and edit the styles and content files inside.
2. pass the path to the theme to the shiroa CLI.

Note since we haven't stablized any things/styles/interfaces on making a theme, so it may be broken in future (but we'll improve the stability in future).