Here is my Stylish to fix this travesty:

@-moz-document domain(stackoverflow.com), domain(superuser.com),
domain(stackexchange.com) {
  #sidebar {
    width: 250px !important;
  }
  .post-text {
    width: 710px !important;
  }
}

Source

I chose 250 because it factors cleanly:

$ factor 250
250: 2 5 5 5

Here is my Stylus [1] to fix this travesty:

@-moz-document
domain(stackexchange.com),
domain(stackoverflow.com),
domain(superuser.com) {
   #sidebar {
      width: 250px !important
   }
   .post-text {
      width: 710px !important
   }
}

I chose 250 because it factors cleanly: 2 * 5 * 5 * 5

1. https://github.com/openstyles/stylus