• Go to this page: My user page
• Click "Create" (or "edit" if there is already a page)
• Copy and Paste this text:

/*Delete this code later if you want to hide spoilers*/
.spoilerbox {
   transition: all 0s; 
   background:#8b0000;
}
.spoilerboxname {
   transition: all 0s; 
   background:#8b0000;
}
.spoilerbutton {
  cursor:arrow;
  visibility:hidden;
}
#message {
    visibility:hidden;
}

• Click "Publish" (It may take a minute to come into effect and the page will have to be reloaded)