<template>
  <div :style="css"></div>
</template>

<script>
export default {
  computed: {
    css () {
      let width = window.innerWidth

      // ... mobile { ... styles }
      // ... desktop { ... styles }

      // ... if width is less than 700, return mobile
      // ... if width greater than 700, return desktop
    }
  }
}
</script>