This has been an issue for quite a while. The "enchantment cost" text in the anvil UI has a dark gray background behind it. This was not the case in earlier versions, at least until 16w39a (video from that version).

This might be intentional because without the darker background, the text is rather ugly and hard to read. However, the dark gray box seems very out of place and until I investigated this a bit more I thought it was just a weird visual bug that happens sometimes, but it seems to happen consistently.

From what I can tell, there's no way to change this background via resource packs either.

To reproduce

1. Put two enchanted books into an anvil
   → The "enchantment cost" text has a dark gray box behind it