revealjs - add style for <kbd> in Revealjs format

[cderv]

Closes #6012

• It adds some styling for kbd element for revealjs. By design, I am choosing the same style than in Bootstrap based document. So it is grey

  • HTML
    
  • Revealjs
    

• It will change based on dark / light theme where the gray background is made darker.

  • While trying, I found it was broken for Bootstrap as the color did not adapt while text color was changing. It looks like this now:
  • HTML with darkly
    
  • Revealjs with dark
    

• Some style can be different between HTML and reveal (like border), but this is because it is depend on the theme used, and both format don't have same themes.

  • This is why kbd can be controlled in revealjs with same variable than in bootstrap

    • quarto-cli/src/resources/formats/revealjs/quarto.scss

      Lines 164 to 169 in c66d1d3

      	// KBD variables
      	$kbd-padding-y: 0.4rem !default;
      	$kbd-padding-x: 0.4rem !default;
      	$kbd-font-size: $presentation-font-size-root !default;
      	$kbd-color: $body-color !default;
      	$kbd-bg: $gray-100 !default; // like in bootstrap style

[cscheid]

This looks good to me!