Closes #961. Replays a previous attempt.

Follow-up to #613. Relatively self-explanatory!

Co-authored-by: Peter Mosses <18308236+pdmosses@users.noreply.github.com>

Added a missing `{: .no_toc }`, to avoid a 3rd-level item in the displayed TOC at the top of the page.

Co-authored-by: Matt Wang <matt@matthewwang.me>

Looking at how the variables are defined in _variables.scss, with various dependencies between them aimed at ensuring a consistent color scheme, one might expect that redefining a given variable would affect the remaining styles that depend on it.

This is not the case, however, due to the order in which the files are processed. This PR edits the documentation to mention the non-propagating behavior of redefined variables, to better guide users that wish to customize the site using custom themes, and call their attention to the need to redefine the dependency relations as well.

Co-authored-by: Matt Wang <matt@matthewwang.me>
Co-authored-by: Patrick Marsceill <pmarsceill@users.noreply.github.com>

These docs outline some of the custom includes that are provided by the theme and can be modified by the user.

Co-authored-by: Matt Wang <matt@matthewwang.me>
Co-authored-by: Patrick Marsceill <pmarsceill@users.noreply.github.com>

(cherry picked from commit dffb2706a158784e2f3091f895a868e373683bc8)

This commit makes the dark mode preview feel a little more interactive
and it also pulls some duplicated code into one file so that people like
me aren't stumped when they are looking at the wrong page!

For pages in utilities/, reorder `nav_order` to follow `parent`.