Halloween

 Halloween  is an annual holiday on October 31st that often inspires holiday themes on websites, both personal and corporate.

See holiday theme for other holidays.

Aaron Parecki
celebrates the entire month of Halloween (October) with:
 * The pixel grid turns orange and black
 * The background map changes to orange theme

capjamesg
celebrates Halloween by adding a spooky banner and changing the emoji in his blog's name to a pumpkin (since Halloween 2021)

Github
Since Halloween 2017 (and possibly earlier?), GitHub changed the colors of their standard contributions calendar visualization from the traditional shades of green to shades of yellow, oranges, and black to celebrate Halloween. Image from 2017:

Twitter
For Halloween, Twitter users will often change their display name to a play on spooky words often along with emoji including pumpkins, ghosts, spiders, webs, or other holiday related emoji. Twitter users also encourage Halloween-specific changes in language, e.g.:
 * Example: Jillian York has documented being quoted in the New York Times as having done this with friends in 2013: https://jilliancyork.com/2013/11/01/thats-yikes-chillian-j-yikes/
 * Examples: wrote about several people who did this on 2017-10-27: Halloween flavored names on Twitter
 * https://twitter.com/ChloeCondon/status/1319830109692411905
 * "For the month of October, plz don't say you have "bugs" in your code, instead say "my code is haunted", thanks!!!! 👻🏚️🎃🦇🦴👩🏼‍💻👻🔮💾" @ChloeCondon October 24, 2020

Letterboxd
For Halloween, Letterboxd changed their three circle logo to give each circle a scary, ghostish face and animate it to appear as if it was slowly dripping blood.