html, body {
  border: 0 none;
  margin: 0;
  padding: 0;
  height: 100%;
  width: 100%;
}
body {
  position: relative;
}
#content {
  width: 100%;
  position: absolute;
  top: 50%;
  transform: translateY(-50%);
}
#bg_normal, #bg_small  {
  width: 100%;
}
#bg_small {
  display: none;
}
#buttons {
  padding-top: 40px;
  text-align: center;
}
#buttons a {
  width: 157px;
  height: 46px;
  display: inline-block;
  background-repeat: no-repeat;
  background-position: center center;
  background-size: contain;
  margin: 0 15px;
}
.brightnessTransition {
  -webkit-transition: all .35s;
  -moz-transition: all .35s;
  transition: all .35s;
}
.brightnessTransition:hover {
  -webkit-filter: brightness(1.2);
  filter: brightness(1.2);
}
#btnApple {
    background-image: url("img/btn_app_store.png");
}
#btnGooglePlay {
    background-image: url("img/btn_google_play.png");
}
#btnWindowsStore {
    background-image: url("img/btn_app_windows_store.png");
}

@media screen and (max-width: 600px) {

  #bg_normal {
    display: none;
  }
  #bg_small {
    display: block;
  }
  #buttons a {
    display: block;
    margin: 20px auto 0 auto;
  }

}