Darken.cs

using UnityEngine;

[ExecuteInEditMode]
public class Darken : MonoBehaviour
{
    Material mat;

    void Start()
    {
        //GetComponent<Camera>().depthTextureMode = DepthTextureMode.Depth;

        CreateMat();
    }

    void CreateMat()
    {
        mat = new Material(Shader.Find("Hidden/Darken"));
    }

    void OnRenderImage(RenderTexture source, RenderTexture destination)
    {
        if (!mat) CreateMat();

        Graphics.Blit(source, destination, mat);
    }
}

Darken.shader

Shader "Hidden/Darken"
{
	Properties
	{
		_MainTex ("Texture", 2D) = "white" {}
	}
	SubShader
	{
		// No culling or depth
		Cull Off ZWrite Off ZTest Always

		Pass
		{
			CGPROGRAM
			#pragma vertex vert
			#pragma fragment frag
			
			#include "UnityCG.cginc"

			struct appdata
			{
				float4 vertex : POSITION;
				float2 uv : TEXCOORD0;
			};

			struct v2f
			{
				float2 uv : TEXCOORD0;
				float4 vertex : SV_POSITION;
			};

			v2f vert (appdata v)
			{
				v2f o;
				o.vertex = mul(UNITY_MATRIX_MVP, v.vertex);
				o.uv = v.uv;
				return o;
			}
			
			sampler2D _MainTex;

			fixed4 frag (v2f i) : SV_Target
			{
				fixed4 col = tex2D(_MainTex, i.uv);
				col = col - 0.1f;
				return col;
			}
			ENDCG
		}
	}
}