The theme_init function is no longer needed, and the example isn't using
it. So we replace it with a bit more description of the theme info in
the comment, and how that works.
Adds another lesson on customising the theme using styles and overriding
templates.
Also some minor language cleanup and add some introductory text before
lesson 1.
To create a derived (or child) theme, the array held in
`App::$theme_info` must contain a key `extends` that contains the name
of the parent theme as a value.
The derived theme tutorial would suggest adding the key in the
`themename_init()` function, however this does not work. The theme info
is repopulated from the theme info file after the theme init function
has run, and so the `extends` key disappears.
This patch adds `extends` as a first class attribute for themes, so that
it can be specified directly in the theme info in the top file comment
block. It also updates the derived theme tutorial to reflect this
change.
