谷歌浏览器开发工具console中报：
Mixed Content: The page at ‘https://www.xxx.cn/wp/wp-login.php’ was loaded over HTTPS, but requested an insecure stylesheet ‘http://www.xxx.cn/wp/wp-admin/load-styles.php?c=0&dir=ltr&load%5B%5D=dashicons,buttons,forms,l10n,login&ver=4.8.2’. This request has been blocked; the content must be served over HTTPS.

nginx虚拟主机中添加一行：
add_header Content-Security-Policy upgrade-insecure-requests;