.parse-text .spoiler-button span {rgba(0,0,0,0.5) url(/img/icons/16x16/spoiler.png) 5px center no-repeat}
↑ NEW


↓ OLD
.parse-text .spoiler-button span {rgba(0,0,0,0.5) url(/img/icons/16x16/spoiler.png) 5px 7px no-repeat}
Fixes the spoiler button not showing centered when it has a smaller font size.