Highlighting Color Scheme and Adding Line Numbers to Expand Shortcodes

[eskopp]

Hello Developer Team,

I am using the Hugo Books theme for my documentation projects and have noticed that the dark highlighting of the expand shortcodes does not ideally suit well-lit environments and various user preferences. Adjusting to a lighter color scheme could enhance readability and better integrate with the usual bright surroundings.

Specific Suggestions

1. Change the background color of the expand areas to a light gray to ensure better readability and visual harmony with the rest of the theme.
2. Implement line numbers within the expandable markdown sections to facilitate navigation and referencing in technical documents.

These changes would significantly improve the working experience with the theme for users who operate in diverse lighting conditions and increase the utility of the expand shortcode.

Thank you for considering these enhancement suggestions!

Best regards,
@eskopp