Pass ShaderLab properties to Standard cginc

This is my first foray into writing shaders so I'll try to make my question as clear as possible.

When writing a barebones shader I know how to pass a ShaderLab value to the cg program thus:

 Shader "Custom/Shader" {
     Properties{
         _Color ("Color", Color) = (1,1,1,1)
     }
     
     SubShader {
         Tags { "RenderType" = "Opaque"}
             CGPROGRAM
         #pragma vertex vert
         #pragma fragment frag
 
         fixed4 _Color;
 
         /* do vertex stuff */
 
         void frag(v2f i) : SV_TARGET {
             return _Color
         }
             ENDCG
     }
  }

However, I want to use the Standard Shader and just tweak it a little bit to get the results I want. The problem is all of the vertex and fragment functions in the Standard shader are buried in cginclude files (UnityStandardCoreForward and UnityStandardCore), and I'm not sure how to pass my ShaderLab properties (ie _Color), to my duplicated copies of those files.

I tried to set it up similar to the above, where I define my value before the #include in the hopes that the value would trickle down, but that's giving me an error:

 fixed4 _Color
 #include "CustomStandardCoreForward.cginc"

...and that's about as far as I got. I'm not sure how to make _Color accessible to vertBase, fragBase, or any of the other functions that are called from the include files.