Thank you for the quick reply.
I am seeing the following mix error on console.
Mixed Content: The page at 'https://www.sample.com/for-dev/' was loaded over HTTPS, but requested an insecure stylesheet 'https://www.sample.com/?display_custom_css=css'. This request has been blocked; the content must be served over HTTPS.
but when I checked the source code
<link rel='stylesheet' id='wp-add-custom-css-css' href='https://www.sample.com/?display_custom_css=css' type='text/css' media='all' />
So, I guess, something wrong with the console…