Adding HTML code?

I use the Zyro builder on a custom domain. I want to edit the index.html (or whatever it is that shows when I do Ctrl-U on my website homepage) so that I can add in a strand of code to make the site more mobile-friendly as well as some CSS to change the colors of a HTML comment plugin. However, I cannot find any file in the File Manager that looks like the HTML for my home page. Where can I add these?

They are like 1.php, 2.php etc.
If you add code directly as soon as you edit your site in the site builder it deletes and overwrites the previously inserted code.