How to change style of single portfolio pages?

Visual Portfolio does nothing with single pages, they are loaded in default way as many other Custom Post Types. You can change it by creating template file in your theme folder for custom post type portfolio:

• https://developer.wordpress.org/themes/template-files-section/page-template-files/
• https://developer.wordpress.org/themes/template-files-section/custom-post-type-template-files/
• https://www.wpbeginner.com/wp-themes/create-custom-single-post-templates-for-specific-posts-or-sections-in-wordpress/