If you need a quick fix, then evaluate the following (or put it into your init.m):
LeviCivitaTensor;
Unprotect[LeviCivitaTensor];
SyntaxInformation[LeviCivitaTensor] = {"ArgumentsPattern" -> {_, _.}};
Protect[LeviCivitaTensor];
Now the highlighter is aware of the possible second argument and everything looks fine:
