Add a not about how color is used

docs/presentations/revealjs/themes.qmd

@@ -200,6 +200,7 @@ Here's a list of all Sass variables (and their default values) used by Reveal th
 |                          | | `caution`   | `#dc3545` |                                                                                                                                        |
 |                          | | `warning`   | `#fd7e14` |                                                                                                                                        |
 |                          | | `important` | `#ffc107` |                                                                                                                                        |
+|                          | Note that style for callout is to have left border using type color, and header background to use a variation of this color.                                       |
 +--------------------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------+
 
 You'll notice that some of the Sass variables use a `presentation-` prefix and some do not. The `presentation-` prefixed variables are specific to presentations, whereas the other variables are the same as ones used for standard Quarto [HTML Themes](/docs/output-formats/html-themes.qmd).