Inspired by this feature request on Meta Science Fiction & Fantasy, this script adds a checkbox to the "question info" sidebar block on each question to allow toggling all spoiler blocks on the page.
Screenshot / Code Snippet:
Here's what it looks like on scifi.stackexchange.com/q/112783 (the screenshots are quite a bit larger than the thumbnails below; click to embiggen):
Download / Install:
Install from GitHub: Install
Requires a UserScript manager (like Greasemonkey or Tampermonkey).
Developed and tested on Windows 10 and Chrome 54. It should work on all desktop operating systems and most browsers (IE version 5.5 or greater, Safari version 4 or greater).
Not tested on mobile.
Any bugs can be reported in comments here, or in GitHub's issue tracker.