Change page theme in Vue 3.

ChangeTheme.vue

<script setup>
import { onMounted, ref } from 'vue';

let th = ref('light');

th.value = localStorage.getItem('theme-xxx-theme') ?? 'light';

function update() {
	document.querySelector('html').setAttribute('color-scheme', th.value);
}

function toggle() {
	th.value = th.value == 'light' ? 'dark' : 'light';
	localStorage.setItem('theme-xxx-theme', th.value);
	update();
}

onMounted(() => {
	update();
});
</script>

<template>
	<div @click="toggle" :class="{ change_theme: true }" :title="$t('Theme light')">
		<div class="theme_icon theme_dark" v-if="th == 'dark'">
			<svg viewBox="0 0 24 24" class="theme-switch-sun">
				<path
					d="M12,18c-3.3,0-6-2.7-6-6s2.7-6,6-6s6,2.7,6,6S15.3,18,12,18zM12,8c-2.2,0-4,1.8-4,4c0,2.2,1.8,4,4,4c2.2,0,4-1.8,4-4C16,9.8,14.2,8,12,8z"
				></path>
				<path
					d="M12,4c-0.6,0-1-0.4-1-1V1c0-0.6,0.4-1,1-1s1,0.4,1,1v2C13,3.6,12.6,4,12,4z"
				></path>
				<path
					d="M12,24c-0.6,0-1-0.4-1-1v-2c0-0.6,0.4-1,1-1s1,0.4,1,1v2C13,23.6,12.6,24,12,24z"
				></path>
				<path
					d="M5.6,6.6c-0.3,0-0.5-0.1-0.7-0.3L3.5,4.9c-0.4-0.4-0.4-1,0-1.4s1-0.4,1.4,0l1.4,1.4c0.4,0.4,0.4,1,0,1.4C6.2,6.5,5.9,6.6,5.6,6.6z"
				></path>
				<path
					d="M19.8,20.8c-0.3,0-0.5-0.1-0.7-0.3l-1.4-1.4c-0.4-0.4-0.4-1,0-1.4s1-0.4,1.4,0l1.4,1.4c0.4,0.4,0.4,1,0,1.4C20.3,20.7,20,20.8,19.8,20.8z"
				></path>
				<path
					d="M3,13H1c-0.6,0-1-0.4-1-1s0.4-1,1-1h2c0.6,0,1,0.4,1,1S3.6,13,3,13z"
				></path>
				<path
					d="M23,13h-2c-0.6,0-1-0.4-1-1s0.4-1,1-1h2c0.6,0,1,0.4,1,1S23.6,13,23,13z"
				></path>
				<path
					d="M4.2,20.8c-0.3,0-0.5-0.1-0.7-0.3c-0.4-0.4-0.4-1,0-1.4l1.4-1.4c0.4-0.4,1-0.4,1.4,0s0.4,1,0,1.4l-1.4,1.4C4.7,20.7,4.5,20.8,4.2,20.8z"
				></path>
				<path
					d="M18.4,6.6c-0.3,0-0.5-0.1-0.7-0.3c-0.4-0.4-0.4-1,0-1.4l1.4-1.4c0.4-0.4,1-0.4,1.4,0s0.4,1,0,1.4l-1.4,1.4C18.9,6.5,18.6,6.6,18.4,6.6z"
				></path>
			</svg>
		</div>
		<div class="theme_icon theme_light" v-if="th != 'dark'" :title="$t('Theme dark')">
			<svg viewBox="0 0 24 24" class="theme-switch-moon">
				<path
					d="M12.1,22c-0.3,0-0.6,0-0.9,0c-5.5-0.5-9.5-5.4-9-10.9c0.4-4.8,4.2-8.6,9-9c0.4,0,0.8,0.2,1,0.5c0.2,0.3,0.2,0.8-0.1,1.1c-2,2.7-1.4,6.4,1.3,8.4c2.1,1.6,5,1.6,7.1,0c0.3-0.2,0.7-0.3,1.1-0.1c0.3,0.2,0.5,0.6,0.5,1c-0.2,2.7-1.5,5.1-3.6,6.8C16.6,21.2,14.4,22,12.1,22zM9.3,4.4c-2.9,1-5,3.6-5.2,6.8c-0.4,4.4,2.8,8.3,7.2,8.7c2.1,0.2,4.2-0.4,5.8-1.8c1.1-0.9,1.9-2.1,2.4-3.4c-2.5,0.9-5.3,0.5-7.5-1.1C9.2,11.4,8.1,7.7,9.3,4.4z"
					stroke-width="2"
					stroke-linecap="round"
					stroke-linejoin="round"
				/>
			</svg>
		</div>
	</div>
</template>

<style scoped>
.change_theme {
	box-sizing: border-box;
	float: right;
	margin: 5px;
	width: 44px;
	height: 44px;
	cursor: pointer;
	background: var(--bg-1);
	border-radius: var(--border-radius);
}
.change_theme .theme_icon {
	box-sizing: border-box;
	position: relative;
	float: left;
	width: 42px;
	height: 42px;
	background: var(--bg-1);
	border-radius: var(--border-radius);
	transition: all 1s;
	border: 1px solid transparent;
}
.change_theme .theme_icon svg {
	position: absolute;
	top: 10px;
	left: 10px;
	width: 20px;
	height: 20px;
	fill: var(--text-1);
}
</style>