Hi Rupesh,
From SAP support, from Uwe, I got the following useful tip so far.
It looks like you build-up the URL by yourself instead using the URL generated by the /UI5/THEME_DESIGNER transaction.
Please use either the transaction or bookmark the following URL:
http://host:port/sap/bc/theming/theme-designer
This will redirect automatically to the right URL adding some URL parameters that the theme designer needs (sap-themeroot, sap-webdav-themeroot). There is a small flaw in the coding that it will not add the parameters if it is called with /index.html.
Using this tip I now indeed get the publish option but it remains greyed out (deactivated) because I have no theming options, the Quick and Expert tab are completely blank, no option to change colors or add a background picture. So I still have some problems to solve.
regards, Remco