If its to style elements within the post content then you can target the category-slug class that is added to the main content. For example:
.single-post .category-your_slug p {
color: red;
}
This would change the content text color of any post with category-your_slug
…. change your_slug to match you category.
If you want to apply CSS to the entire page, you would need to add a little PHP to include the category-slug in the body tag. I provide that PHP Snippet here:
https://generatepress.com/forums/topic/change-header-background-for-category-or-pages/page/2/#post-1460013