Does everything work as expected?
Do you have the newest version of the plugin (0.2.2-1)?
I can't reproduce the error and since there is no use of CSS within emoji-extended anymore...
As long as the compatibility hash is added to a v0.5.x compatible version of the plugin and published to npm, then you can freely publish the v0.6.x-compatible version.
NodeBB v0.5.4 and onwards will query the NodeBB Package Manager for the suggested package version to download.