Passing props that will be used in style in child component - Vue

I have a button that runs the function startProcess which will generate a random number. This random number will be passed as a prop to Child.vue that will be used in style. I looked into some pages "How to use props in style in vue". The solution was to use computed, but nothing seems to work. For a better understanding, please check the code.

P.S. This is a simplified code. Removed template, script, style.

App.vue

<button @click="startProcess">Start</button>
<Child v-if="toggleChild" :top="top" />

data() {
    return {
        toggleChild: false,
        top: 0
    }
},
methods: {
    startProcess() {
        this.toggleChild = !this.toggleChild;
        this.top = Math.random();
    };
}

Child.vue

<button @click="logTop">Log</button>

props: { top: Number },
computed: {
    return {
        cssProps() {
            "--top": `${this.top}%`;
        };
    };
};

.foo {
  top: var(--top);
};

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