antlr-font-lock-maximum-decoration is a variable defined in `antlr-mode.el'.
Its value is

The maximum decoration level for fontifying actions.
Value `none' means, do not fontify actions, just normal grammar code
according to `antlr-font-lock-additional-keywords'. Value `inherit'
means, use value of `font-lock-maximum-decoration'. Any other value is
interpreted as in `font-lock-maximum-decoration' with no level-0
fontification, see `antlr-font-lock-keywords-alist'.

While calculating the decoration level for actions, `major-mode' is
bound to `antlr-language'. For example, with value
((java-mode . 2) (c++-mode . 0))
Java actions are fontified with level 2 and C++ actions are not
fontified at all.

You can customize this variable.