docs: Mention caveat about variable dependencies (#555)

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>