CSS变量与背景图像网址

我误解了css变量的功能吗？我试图将背景图像URL传递给这样的变量:当我传递像颜色等简单的东西时,它似乎工作得很好......

:root {
  --slide-1: url(/static/images/slideshow/slide1.jpg) ;
  --slide-2: url(/static/images/slideshow/slide2.jpg) ;
  --slide-3: url(/static/images/slideshow/slide3.jpg) ;
  --slide-4: url(/static/images/slideshow/slide4.jpg) ;
  --slide-5: url(/static/images/slideshow/slide5.jpg) ;
}

//and then

.jumbotron > .jumbotron-slideshow:nth-child(1) { 
  background-image: var(--slide-1); 
}

.jumbotron > .jumbotron-slideshow:nth-child(2) {
  animation-delay: 6s;
  background-image: var(--slide-2);
}

.jumbotron > .jumbotron-slideshow:nth-child(3) {
  animation-delay: 12s;
  background-image: var(--slide-3);
}

.jumbotron > .jumbotron-slideshow:nth-child(4) {
  animation-delay: 18s;
  background-image: var(--slide-4);
}

.jumbotron > .jumbotron-slideshow:nth-child(5) {
  animation-delay: 24s;
  background-image: var(--slide-5);
}