Be aware that everything that is advertised within the Porto theme website pages is not easily customizable to WCAG standards without knowing CSS and having your editors going into the code and knowing what they are doing or, I think, going into the theme, copying the theme (which may bring up issues with upgrades) and changing it there.

According to Mandeeps: Shortcodes are not tested for accessibility on the http://porto.mandeeps.com/features/accessibility

Note: Only the core theme and its components are accessible. The included templates and shortcodes are not tested for accessibility. Ultimately, the content editor is responsible for adding content to a page that is accessible to all users.

This could be fixed to a degree if the the toggle dynamically adds preview-active class, that is currently hard coded color (#999) (WAY too light to meet ANY standard, dark or light) could be switched in style switcher.