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Generate Sky & Sun in Ray Marching algorithm is very simple to process.
When the map procedure don’t intersect a scene obiect (In my shader Hit variable is false) gl_FragColor set to background color.
Is simple way to create it:

  1. First define te Sun structure: 
    struct Sun {
	vec3 posistion;
	vec3 color;
	float haloSize;
	float haloPower;
	float discSize;
	float discPower;
	float discArea;
	float horizontal;
};
Sun sunLight=Sun(vec3(-0.2, 0.2,  -0.4),vec3(1.0,1.0,1.0), 100.0, 0.3, 100.0, 1.5, 0.9, 5.0);
  2. Test the intersect a scene object: 
    	Object sceneObject=rayMarching(ray, rayDir);
	vec3 outColor=vec3(0.0, 0.0, 0.0);
	if (sceneObject.hit){
...
		outColor=outColor * lightColor * baseShadow + sceneObject.mat.ambient;
	} else {
		outColor=getSky(sceneObject, ray, rayDir);	
	}
 
	gl_FragColor=vec4(outColor.r, outColor.g, outColor.b, 1.0);
  3. If intersect is false result generate background color in the getSky function: 
    vec3 getSky(Object sceneObject, vec3 ray, vec3 rayDir){
 
	vec3 skyColor=vec3(0.0);
	float horizontHeight = pow(1.0-max(rayDir.y,0.0), sunLight.horizontal);
	skyColor=mix(vec3(0.0, 0.0, 0.5), vec3(1.0, 0.0, 0.0), horizontHeight);
 
	#if ENABLE_SUN==1
		float sunAmount = max(dot(rayDir, normalize(sunLight.posistion)), 0.0);
		// Sun Halo
		skyColor = skyColor + sunLight.color * pow(sunAmount, sunLight.haloSize) * sunLight.haloPower;
		// Sun Disc
		skyColor = skyColor + sunLight.color * min(pow(sunAmount, sunLight.discSize) * sunLight.discPower, sunLight.discArea);
	#endif
 
	return clamp(skyColor, 0.0, 1.0);
}
  4. To create more funny effect – where intersect if true, create a halo effect in post processing (in future).

Example of this sun effect: