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HTML 

              
                <div class="sky">
  <div class="cloud s1"></div>
	<div class="cloud s2"></div>
	<div class="cloud s3"></div>
	<div class="cloud s4"></div>
	<div class="cloud s5"></div>
  <div class="star-wrapper">
    <div class="stars small"></div>
    <div class="stars medium"></div>
    <div class="stars large"></div>
  </div>
</div>
!

CSS 

              
                .sky {
  width: 100vw;
  height: 100vh;
  position: fixed;
	top: 0;
	right: 0;
	bottom: 0;
	left: 0;
  .cloud {
    width: 22.5em;
    height: 7.5em;
    background: #f2f9fe;
    -moz-border-radius: 6.25em;
    -webkit-border-radius: 6.25em;
    border-radius: 6.25em;
    position: relative;
    margin: 7.5em auto 1.25em;
  }
  .cloud:after, .cloud:before{
    content: "";
    position: absolute;
    background: #f2f9fe;
    z-index: -1;
    border-radius: 50%;
  }
  .cloud:after{
    width: 7.5em;
    height: 7.5em;
    top: -3.125em;
    left: 3.125em;
  }
  .cloud:before{
    width: 11.25em;
    height: 11.25em;
    top: -5.625em;
    right: 3.125em;
  }
  .cloud.s1 {
    transform: scale(0.9, 0.9);
    animation: moveclouds 30s linear infinite;
  }
  .cloud.s2{
    left: 12.5em;
    transform: scale(0.7, 0.7);
    animation: moveclouds 50s linear infinite;
  }
  .cloud.s3{
    left: -15.625em;
    top: -12.5em;
    transform: scale(0.4, 0.4);
    animation: moveclouds 40s linear infinite;
  }
  .cloud.s4{
    left: 29.375em;
    top: -35.65em;
    transform: scale(0.5, 0.5);
    animation: moveclouds 36s linear infinite;
  }
  .cloud.s5{
    left: -38em;
    top: -38em;
    transform: scale(0.3, 0.3);
    animation: moveclouds 44s linear infinite;
  }
}
@keyframes moveclouds {
	0%{
		margin-left: 125%;
	}
	100%{
		margin-left: -125%;
	}
}

@function box-shadow($stars) {
	$bxshadow: ();
	@for $i from 1 to $stars {
		$bxshadow: append($bxshadow, (random(2000) + 0px) (random(2000) + 0px) #fff, comma);
	}
	@return unquote($bxshadow);
}

$bxshadow-large: box-shadow(300);
$bxshadow-medium: box-shadow(500);
$bxshadow-small: box-shadow(800);
.star-wrapper {
  display: none;
  position: absolute;
  top: 0;
  left: 0;
}
.stars.large {
	height: 3px;
	width: 3px;
	background: transparent;
	box-shadow: $bxshadow-large;
	animation: blink-large 7s infinite;
}
.stars.medium {
	height: 2px;
	width: 2px;
	background: transparent;
	box-shadow: $bxshadow-medium;
	animation: blink-medium 5s infinite;
}
.stars.small {
	height: 1px;
	width: 1px;
	background: transparent;
	box-shadow: $bxshadow-small;
	animation: blink-small 3s infinite;
}

@keyframes blink-large {
	0% {
		box-shadow: $bxshadow-large;
	}	
	70% {
		$colors: '#fff', 'transparent';
		$stars: $bxshadow-large;
		@for $i from 1 to length($stars) {
			$star: set-nth(nth($stars, $i), 3, unquote(nth($colors, random(length($colors)))));
			$stars: set-nth($stars, $i, $star);
		}
		box-shadow: $stars;
	}
	100% {
		box-shadow: $bxshadow-large;
	}
}
@keyframes blink-medium {
	0% {
		box-shadow: $bxshadow-medium;
	}	
	70% {
		$colors: '#fff', 'transparent';
		$stars: $bxshadow-medium;
		@for $i from 1 to length($stars) {
			$star: set-nth(nth($stars, $i), 3, unquote(nth($colors, random(length($colors)))));
			$stars: set-nth($stars, $i, $star);
		}
		box-shadow: $stars;
	}
	100% {
		box-shadow: $bxshadow-medium;
	}
}
@keyframes blink-small {
	0% {
		box-shadow: $bxshadow-small;
	}	
	70% {
		$colors: '#fff', 'transparent';
		$stars: $bxshadow-small;
		@for $i from 1 to length($stars) {
			$star: set-nth(nth($stars, $i), 3, unquote(nth($colors, random(length($colors)))));
			$stars: set-nth($stars, $i, $star);
		}
		box-shadow: $stars;
	}
	100% {
		box-shadow: $bxshadow-small;
	}
}
!

JS 

              
                /** checks the time of day at regular intervals **/
class DayState {
  /**
   * day state initialization
   */
  constructor() {
    this.timer();
    setInterval(() => this.timer(), 60000);
  }
  /**
   * check day state
   */
  timer() {
    this.skyState();
    this.starState();
  }
  // set the skys state depending on time of day
  skyState() {
    var sky = document.querySelector('.sky');
    var time = new Date();
    var hour = time.getHours();
    if(hour >= 7 && hour < 16) {
      sky.style.background = 'linear-gradient(180deg, #2980b9, #6dd5fa)';
    } else if(hour >= 16 && hour < 18) {
      sky.style.background = 'linear-gradient(180deg, #659999, #f4791f)';
    } else {
      sky.style.background = 'radial-gradient(ellipse at bottom, rgb(9,76,114) 0%,rgb(1,41,63) 100%)';
    }
  }
  // set the stars state depending on time of day
  starState() {
    var stars = document.querySelector('.star-wrapper');
    var time = new Date();
    var hour = time.getHours();
    if(hour >= 17 || hour < 8) {
      stars.style.display = 'block';
    } else {
      stars.style.display = 'none';
    }
  }
}
/* Create new instance of DayState checker */
let clock = new DayState();
!
999px

Console