Add Halloween effects to Blogger

Blogger lets you edit the theme HTML directly, which is all you need.

Steps

Open your blog, go to Theme and use the arrow next to Customize to choose Edit HTML.
Find the closing </body> tag near the end of the file.
Paste the snippet just above it and click Save.

The snippet

One paste, no other files. It loads halloween.js 1.0.3 from jsDelivr and turns on webs in two corners, blinking eyes, flying witches, a dropping spider and a rising tombstone.

<script src="https://cdn.jsdelivr.net/npm/halloween.js@1.0.3/dist/halloween.iife.js"></script>
<script>
(function () {
  function apply() {
    document.body.classList.add("halloween", "halloween-screen-left-top", "halloween-screen-right-bottom", "halloween-eyes", "halloween-witches", "halloween-spider-drop", "halloween-tombstones");
  }
  if (document.readyState === "loading") {
    document.addEventListener("DOMContentLoaded", apply);
  } else {
    apply();
  }
})();
</script>

Make it yours

Pick corners, effects, intensity, color and season dates in the playground and open the Paste-only tab. It produces this same snippet, library line included, with your choices.

Good to know

The effects only appear in a window around Halloween, 18 October to 2 November by default. Change the dates in the playground to see them on another day.
Ambient effects stop when a visitor has reduced motion turned on. The screen-corner webs stay.
The effects sit in a fixed layer and never shift your page layout.
Back up your theme first (Theme → Backup) before editing its HTML.
Nothing showing? Reload without cache, then check that the snippet is on the live site and that no content blocker is stripping it.

Other platforms

All options on the halloween.js home page, full documentation on GitHub.