Fix dark mode rendering of Cheatsheet examples

[julien-deramond]

Description

This PR fixes the dark mode rendering of Cheatsheet examples.
I chose to apply minimal changes to make it work rather well: using the body color and some grays working both in light and dark modes.

Type of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist

• I have read the contributing guidelines
• My code follows the code style of the project (using npm run lint)
• My change introduces changes to the documentation
• I have updated the documentation accordingly
• (N/A) I have added tests to cover my changes
• All new and existing tests passed

Live previews

• https://deploy-preview-38905--twbs-bootstrap.netlify.app/docs/5.3/examples/cheatsheet/ (light and dark modes)
• https://deploy-preview-38905--twbs-bootstrap.netlify.app/docs/5.3/examples/cheatsheet-rtl/ (light and dark modes)

Related issues

Closes #38904