If you were visiting the Styles section to modify Style Properties, please note:
It is not currently stated anywhere, but this section is essentially deprecated, at least in XenForo. XenForo's built-in Style Properties manager is more powerful than the one packaged with VaultWiki's Admin Panel, which was built mainly with vBulletin 3 in mind, and to handle features that vBulletin overall was ill-equipped for. These issue does not exist in XenForo's Style Properties system, so you should use that if you can.
That said, I will look into this error so that at least it is resolved while the Styles section is available.