Basic shader that makes things dark and blueish. The Vertex shader is just a generic one. The fragment shader gets the lightness of the pixel at the right place on the texture, darkens it, and gives it a blue tint.

Alternative, for ShaderToy.

float lightDistance = 100.0;

void main(void)
{
	vec2 uv = gl_FragCoord.xy / iResolution.xy;
    uv.y = iResolution.y - uv.y;
    vec4 tex = texture2D(iChannel0, uv);
    
    // How far are we from the mouse?
    float mouseDist = distance(gl_FragCoord.xy, iMouse.xy);
    
    if (mouseDist > lightDistance) {
    
    	// How "light" is this pixel?
    	float lightness = (tex.r + tex.g + tex.b) / 3.0;
    
    	// Make everything dark and blue-ish
    	gl_FragColor = vec4(lightness/5.0, lightness/5.0, lightness/3.0, 1.0);
    } else {
        gl_FragColor = tex;
    }
}

night.fragment.glsl

#ifdef GL_ES
precision mediump float;
#endif

varying vec4 v_color;
varying vec2 v_texCoords;
uniform sampler2D u_texture;

void main()
{
	vec4 tex = v_color * texture2D(u_texture, v_texCoords);

	float lightness = (tex.r + tex.g + tex.b) / 3.0;

	// Make things dark and blueish
	gl_FragColor = vec4(lightness/5.0, lightness/5.0, lightness/3.0, tex.a);
}

night.vertex.glsl

attribute vec4 a_position;
attribute vec4 a_color;
attribute vec2 a_texCoord0;

uniform mat4 u_projTrans;

varying vec4 v_color;
varying vec2 v_texCoords;

void main()
{
	v_color = a_color;
	v_texCoords = a_texCoord0;
	gl_Position =  u_projTrans * a_position;
}