In vue.js component, how to use props in css?

You actually can!

You should define the CSS variables in a Computed Property, then call the computed property as a style attribute to the element that will require the CSS variable, and finally you may use the variable within the tags at the bottom of your document.

new Vue({
  el: '#app',
  data: function() {
    return {
      baseFontSize: 1,
      bgHoverColor: "#00cc00",
      hoverContent: "Hovering!"
    }
  },
  computed: {
    cssProps() {
      return {
        '--hover-font-size': (this.baseFontSize * 2) + "em",
        '--bg-hover-color': this.bgHoverColor,
        '--hover-content': JSON.stringify(this.hoverContent)
      }
    }
  }
})

div {
  margin: 1em;
}

div.test:hover {
  background-color: var(--bg-hover-color);
  font-size: var(--hover-font-size);
}

div.test:hover::after {
  margin-left: 1em;
  content: var(--hover-content);
}

<script src="https://unpkg.com/vue/dist/vue.js"></script>

<div id="app" :style="cssProps">

  <div>Hover text: <input type="text" v-model="hoverContent"></div>
  <div>Hover color: <input type="color" v-model="bgHoverColor"></div>

  <div class="test">Hover over me</div>
</div>

Or have a look here: https://codepen.io/richardtallent/pen/yvpERW/
And here: https://github.com/vuejs/vue/issues/7346

You don't. You use a computed property and there you use the prop to return the style of the div, like this:

<template>
  <div id="a" :style="style" @mouseover="mouseOver()">
  </div>
</template>

<script>
  export default {
    name: 'SquareButton',
    props: ['color'],
    computed: {
      style () {
        return 'background-color: ' + this.hovering ? this.color: 'red';
      }
    },
    data () {
      return {
        hovering: false
      }
    },
    methods: {
      mouseOver () {
       this.hovering = !this.hovering
      }
    }
  }
</script>

<style scoped>
<style>

As we are in 2020 now, I suggest using this trick with a css function called var

<template>
    <div id="a" :style="cssVars"></div>
</template>

<script>
export default {
    props: ['color'],
    computed: {
      cssVars () {
        return{
          /* variables you want to pass to css */
          '--color': this.color,
        }
    }
}
<script>

<style scoped>
#a{
    background-color: var(--color);
}
</style>

This method is very useful because it allows you to update the passed values through css later on (for example when you apply hover event).

credit

If you need css that can't be applied by a style attribute like pseudo classes or media queries, what I do is the following:

Create a globally available style component when initializing Vue (you need it as otherwise you run into linting issues). It creates a style tag that simply renders the content in the slot:

I would only use this if you really need both dynamic values in your css and css features that can't be applied to a style attribute.

import Vue from 'vue'
import App from './App.vue'
import router from './router'
import store from './store'

Vue.config.productionTip = false
Vue.component('v-style', {
  render: function(createElement) {
    return createElement('style', this.$slots.default)
  }
})

new Vue({
  router,
  store,
  render: h => h(App)
}).$mount('#app')

Then use it at the top of your template like this and you get the full JavaScript scope of your component and the full css syntax combined:

<template>
  <v-style>
    @media screen and (max-width: 820px) {
      .gwi-text-media-{{ this.id }} {
        background-image: url({{ mobileThumb }});
      }
    }
  </v-style>
</template>

It seems a bit hacky to me, but it does it's job and I would rather go like this in some cases than having to add additional JS for mouse-over or resize events that have a big potential to slow down your application performance.

Why not just use :style prop in this way:

<template>
  <div :style="{ backgroundColor: color }">
</template>

<script>
export default {
  props: {
    color: {
      type: String,
      default: ''
    }
  }
}
</script>

Make sure you define css properties in camelCase style.