Fix mixed theme state after sign-in route transition

packages/harmony/src/foundations/theme/ThemeProvider.tsx

@@ -1,4 +1,4 @@
-import { useEffect, type ReactNode } from 'react'
+import { useEffect, useRef, type ReactNode } from 'react'
 
 import { ThemeProvider as EmotionThemeProvider } from '@emotion/react'
 
@@ -13,14 +13,51 @@ type ThemeProviderProps = {
   children: ReactNode
 }
 
+type ThemeProviderInstance = {
+  id: number
+  theme: Theme
+}
+
+let instanceId = 0
+let providerStack: ThemeProviderInstance[] = []
+
+const syncDocumentTheme = () => {
+  if (typeof document === 'undefined') return
+
+  const activeProvider = providerStack[providerStack.length - 1]
+  if (activeProvider) {
+    document.documentElement.setAttribute('data-theme', activeProvider.theme)
+  } else {
+    document.documentElement.removeAttribute('data-theme')
+  }
+}
+
 export const ThemeProvider = (props: ThemeProviderProps) => {
   const { children, theme } = props
+  const providerIdRef = useRef<number | null>(null)
+  if (providerIdRef.current == null) {
+    providerIdRef.current = ++instanceId
+  }
+  const providerId = providerIdRef.current
 
   useEffect(() => {
-    if (typeof document !== 'undefined') {
-      document.documentElement.setAttribute('data-theme', theme)
+    providerStack = [...providerStack, { id: providerId, theme }]
+    syncDocumentTheme()
+
+    return () => {
+      providerStack = providerStack.filter(
+        (provider) => provider.id !== providerId
+      )
+      syncDocumentTheme()
     }
-  }, [theme])
+  }, [providerId])
+
+  useEffect(() => {
+    providerStack = providerStack.map((provider) =>
+      provider.id === providerId ? { ...provider, theme } : provider
+    )
+    syncDocumentTheme()
+  }, [providerId, theme])
 
   return (
     <EmotionThemeProvider theme={themes[theme]}>