Переписал на vue3

src/composables/useTheme.js

@@ -0,0 +1,30 @@
+import { ref, computed, onMounted } from 'vue'
+
+export function useTheme() {
+  const theme = ref('dark')
+
+  const isLightTheme = computed(() => theme.value === 'light')
+
+  function applyTheme(name) {
+    theme.value = name
+    document.documentElement.className = name
+    localStorage.setItem('theme', name)
+  }
+
+  function toggleTheme() {
+    applyTheme(theme.value === 'dark' ? 'light' : 'dark')
+  }
+
+  onMounted(() => {
+    const saved = localStorage.getItem('theme')
+    if (saved === 'dark' || saved === 'light') {
+      applyTheme(saved)
+      return
+    }
+
+    const prefersDark = window.matchMedia('(prefers-color-scheme: dark)').matches
+    applyTheme(prefersDark ? 'dark' : 'light')
+  })
+
+  return { theme, isLightTheme, toggleTheme }
+}