Add documentation for SCSS variables in revealjs

Author: cderv

docs/presentations/revealjs/themes.qmd

@@ -176,6 +176,32 @@ Here's a list of all Sass variables (and their default values) used by Reveal th
 | `$presentation-slide-text-align`       | left                                     |
 | `$presentation-title-slide-text-align` | center                                   |
 
+### Callouts
+
++--------------------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------+
+| Variable                 | Notes                                                                                                                                                              |
++==========================+====================================================================================================================================================================+
+| `$callout-border-width`  | The left border width of callouts. Defaults to `0.3rem`.                                                                                                           |
++--------------------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------+
+| `$callout-border-scale`  | The border color of callouts computed by shifting the callout color by this amount. Defaults to `0%`.                                                              |
++--------------------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------+
+| `$callout-icon-scale`    | The color of the callout icon computed by shifting the callout color by this amount. Defaults to `10%`.                                                            |
++--------------------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------+
+| `$callout-margin-top`    | The amount of top margin on the callout. Defaults to `1rem`.                                                                                                       |
++--------------------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------+
+| `$callout-margin-bottom` | The amount of bottom margin on the callout. Defaults to `1rem`.                                                                                                    |
++--------------------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------+
+| `$callout-color-<type>`  | The colors for the various types of callouts. Defaults:                                                                                                            |
+|                          |                                                                                                                                                                    |
+|                          | | type        | default   |                                                                                                                                        |
+|                          | |-------------|-----------|                                                                                                                                        |
+|                          | | `note`      | `#0d6efd` |                                                                                                                                        |
+|                          | | `tip`       | `#198754` |                                                                                                                                        |
+|                          | | `caution`   | `#dc3545` |                                                                                                                                        |
+|                          | | `warning`   | `#fd7e14` |                                                                                                                                        |
+|                          | | `important` | `#ffc107` |                                                                                                                                        |
++--------------------------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------+
+
 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).
 
 Since all Quarto themes use the same Sass format, you can use a single theme file for both HTML / website documents and presentations.