Add .include-dark and fix fig-alt for reader-mode screenshot

Author: cderv

docs/websites/website-navigation.qmd

@@ -473,7 +473,7 @@ navbar: false
 
 If you'd like users to be able to hide the side navigation and table of contents and have a more focused reading experience, you can enabled `reader-mode`. When enabled, a `reader-mode` toggle will appear on the navbar, if present, or on the sidebar. When pressed, the toggle will 'roll up' the sidebar and table of contents.
 
-![](images/reader-mode.png){fig-alt="Reader mode toggle appearing the top navigation."}
+![](images/reader-mode.png){.include-dark fig-alt="Reader mode toggle appearing in the top navigation with the table of contents collapsed into an 'On this page' dropdown."}
 
 To enable `reader-mode`, use the following in your project: