link to layering explanation

Author: cscheid

docs/presentations/revealjs/themes.qmd

@@ -215,3 +215,4 @@ See these articles to learn more about using Reveal:
 -   [Reveal Basics](index.qmd) covers the basic mechanics of creating presentations.
 -   [Presenting Slides](presenting.qmd) describes slide navigation, printing to PDF, drawing on slides using a chalkboard, and creating multiplex presentations.
 -   [Advanced Reveal](advanced.qmd) delves into transitions, animations, advanced layout and positioning, and other options available for customizing presentations.
+-   [More About Quarto Themes](docs/output-formats/html-themes-more.qmd) describes the layering system in more detail, including interactions with other SCSS files and [`_brand.yml`](/docs/authoring/brand.qmd).