The following productions have trailing ; that w3c/webref currently includes in their value definitions:

<custom-selector> = <custom-arg>? : <extension-name> [ ( <custom-arg>+#? ) ]? ;
<custom-arg> = $ <ident-token> ;

$ should be annotated between quotes, except if you plan to define it in CSS Values as one of the literal values that can appear without quotes.

Some property value definitions also include the slash (/), the comma (,), and/or parentheses as literals. These represent their corresponding tokens. Other non-keyword literal characters that may appear in a component value, such as “+”, must be written enclosed in single quotes.

Similarly, <custom-arg>+#? should be [<custom-arg>+#]?, or +#? should be defined as a valid muliplier stack in CSS Values.

I think <extension-name> can be defined as a <dashed-ident>:

An <extension-name> is any identifier that starts with two dashes (U+002D HYPHEN-MINUS), like --foo, or even exotic names like -- or ------.