Reason

Cosmo HTML template contains nice background effects in the intro section and javascript plugin “Particles.js” is responsible for its proper functionality. When you try to open HTML files locally (for example, by simply double-click on index.html) the majority of modern browsers won’t allow Particles.js configuration file to be loaded particlesjs-config.json due to security restrictions for cross-origin requests. Thereafter the plugin can’t be initialized properly without those configuration files

Solution

Just use a web-server (local or remote) for the template preview. This way Particles.js will work properly and the special effects will appear on index page.