Support selecting theme on the footer (#35741)

Fixes: https://github.com/go-gitea/gitea/pull/27576

modules/web/middleware/cookie.go

@@ -11,6 +11,7 @@ import (
 
 	"code.gitea.io/gitea/modules/session"
 	"code.gitea.io/gitea/modules/setting"
+	"code.gitea.io/gitea/modules/util"
 )
 
 // SetRedirectToCookie convenience function to set the RedirectTo cookie consistently
@@ -39,11 +40,13 @@ func SetSiteCookie(resp http.ResponseWriter, name, value string, maxAge int) {
 	// These are more specific than cookies without a trailing /, so
 	// we need to delete these if they exist.
 	deleteLegacySiteCookie(resp, name)
+
+	// HINT: INSTALL-PAGE-COOKIE-INIT: the cookie system is not properly initialized on the Install page, so there is no CookiePath
 	cookie := &http.Cookie{
 		Name:     name,
 		Value:    url.QueryEscape(value),
 		MaxAge:   maxAge,
-		Path:     setting.SessionConfig.CookiePath,
+		Path:     util.IfZero(setting.SessionConfig.CookiePath, "/"),
 		Domain:   setting.SessionConfig.Domain,
 		Secure:   setting.SessionConfig.Secure,
 		HttpOnly: true,