styles.css

* {
  /* CSS custom properties (use: var(--custom-property);) */
  --primary-color: #5b81b2;
  --light-theme-background: #eee;
  --header-height: 56px;
  --shadow-2dp: 0 2px 2px 0 rgba(0, 0, 0, 0.14),
                0 1px 5px 0 rgba(0, 0, 0, 0.12),
                0 3px 1px -2px rgba(0, 0, 0, 0.2);
  ;
  --shadow-4dp: 0 4px 5px 0 rgba(0, 0, 0, 0.14),
                0 1px 10px 0 rgba(0, 0, 0, 0.12),
                0 2px 4px -1px rgba(0, 0, 0, 0.4);
  box-sizing: border-box;
}

html, body {
  width: 100vw;
  height: 100vh;
  margin: 0;
  padding: 0;
  font-family: 'Titillium Web', sans-serif;
  overflow: hidden;
}

/*flexbox layout*/
body {
  display: flex;
  flex-direction: column;
  background: var(--light-theme-background);
}

/*header*/
body header {
  width: 100%;
  padding: 0 16px;
  background-color: var(--primary-color);
  color: #fff;
  font-size: 20px;
  height: var(--header-height);
  line-height: var(--header-height);
  box-shadow: var(--shadow-4dp);
  z-index: 1;
}

/*main container of content*/
body main {
  flex: 1;
  overflow-y: auto;
}

/*youtube video container*/
body .yt-container {
  display: flex;
  justify-content: center;
}
/*next video countdown*/
body .countdown {
  position: absolute;
  right: 0;
  top: 72px;
  padding: 8px 24px 8px 32px;
  background: #D50000;
  color: #fff;
  border-radius: 4px 0px 0px 16px;
  transition: all ease-out 0.7s;
  opacity: 1;
  transform: none;
  will-change: transform;
  z-index: 2;
}
body .countdown.hidden {
  opacity: 0;
  transform: translateX(100%);
}

/*detail with video info and cheatsheets*/
body .details {
  flex: 1;
  flex-direction: column;
  display: flex;
}
/*info card with views and add to favorites*/
body .details .info {
  display: flex;
  width: 978px; /*video width*/
  margin: 16px;
  align-self: center;
  background: #fff;
  border-radius: 2px;
}
body .details .info .view-count {
  flex: 1;
  font-size: 18px;
  margin: 16px;
  font-weight: bold;
  color: #6c6c6c;
  line-height: 37px;
}

/*cheatsheet images gallery*/
body .details .image-gallery {
  flex: 1;
  display: flex;
  overflow-x: auto;
  overflow-y: visible;
  padding-bottom: 8px;
  cursor: move;
  cursor: -webkit-grab;
  cursor: grab;
}
/*cursor for scrolling/grabbing*/
body .details .image-gallery.grabbing {
  cursor: move;
  cursor: -webkit-grabbing;
  cursor: grabbing;
}
/*image styling*/
body .details .image-gallery img {
  height: 100%;
  max-height: 250px;
  margin: 0 8px;
  box-shadow: var(--shadow-2dp);
  border-radius: 2px;
  -webkit-user-drag: none;
  -khtml-user-drag: none;
  -moz-user-drag: none;
  -o-user-drag: none;
  user-drag: none;
}
body .details .image-gallery .image-container {
  position: relative;
}
/*icon to jump to video part*/
body .details .image-gallery .image-container .jump-to-icon {
  color: #fff;
  position: absolute;
  top: 8px;
  right: 16px;
  opacity: 0;
  -webkit-user-select: none;
  -khtml-user-select: none;
  -moz-user-select: none;
  -o-user-select: none;
  user-select: none;
}
body .details .image-gallery .image-container:hover .jump-to-icon {
  opacity: 1;
}

body .details .image-gallery .image-container .jump-to-icon:focus {
  opacity: 1;
}
/*general styling for buttons*/
button.fav {
  background-color: var(--primary-color);
  text-transform: uppercase;
  color: #fff;
  border-radius: 2px;
  border: none;
  font-size: 18px;
  padding: 8px;
  margin: 16px;
  box-shadow: var(--shadow-2dp);
  cursor: pointer;
}

button.pseudo {
  border: none;
  background: none;
}

/*youtube player on desktop*/
#ytplayer {
	height: 550px;
	width: 978px;
}

/*responsive design from 1100px e.g. tablets and mobile phones*/
@media only screen and (max-width: 1100px) {
	/* mobile */

  /*aspect ratio 16:9*/
	#ytplayer {
		width: 100vw;
    height: 56.25vw; /*vw to use ar!*/
		padding: 0;
	}

  body header {
    font-size: 18px;
    height: 56px;
  }

	body .details .info {
	  display: flex;
	  width: 100%;
	  margin: 0;
	}

  body .details .image-gallery {
    margin: 32px 0;
    padding: 0 16px;
  }

  body .details .image-gallery .img-container {
    max-height: 300px;
    margin: 0 16px;
  }

  body .details .image-gallery .img-container:last-of-type {
    /*Chrome bugfix*/
    padding-right: 16px;
  }

  body .details .image-gallery .image-container .jump-to-icon {
    opacity: 1;
  }

  button.fav {
    font-size: 16px;
  }
}

/*some additional responsive styles for mobile phones*/
@media only screen and (max-width: 600px) {
	/* mobile */
  body header {
    font-size: 14px;
  }

  body .details .image-gallery .img-container {
    max-height: 180px;
    margin: 0 8px;
  }
}

body .details .image-gallery.vertical-scrolling {
  flex-wrap: wrap;
  justify-content: center;
}