ożywiona zmiana klasy kątowej

<body [ngClass]="{'short': condition, 'long': !condition }">
  ...
</body>

.short {
  background-color: blue;
  transition: all 0.5s 0s ease-in;
}
.long {
  background-color: red;
  transition: all 1s 0s ease-in;
}
MitchAloha