Happens on both light and dark. Looking at other people's screenshots, it doesn't seem to be an issue for everyone. Windows 8.1 x64 Firefox 38.0 Spoiler: screenshots
This was actually a problem with software on my computer that affected my font rendering, not a problem with the site. This can be ignored.
It's probably because the arrow is actually fontawesome character and not a CSS arrow. The latter is quite finicky and not very extensible. If you use a browser add-on like Stylish I can help you remove the drop shadow for the arrows.
