Hello,
You are using WordPress, what index.html are you talking about? XStore is WordPress theme it uses PHP, JS and CSS files, it’s not HTML theme. If you need to add any custom script to before head section or in footer you can use Insert headers and Footers plugin.
If you need to add some piece of simple HTML code (without scripts or PHP) to your pages you can use either page content editor and Raw HTML element or footer widgets or footer Static block depending on the place where exactly you need to show that HTML.
Regards