On category.html, configure the gray background-color as an active link to its subcategory page

[ArthurSmid]

As on the index.html (notice the background-color is an active link and has the hover-over effect too https://little-help-book.netlify.app/table-of-contents-style-homepage/index.html), add this to the subcategory names' gray background-color on the category.html.