Hi @madan,
This kind of notice most often comes from plugins (and not from the theme) and are generally not dangerous at all for your site (notice or warning).
The “DOB” notice alert (fatal), may come from a misconfiguration (Required plugin like BuddyPress is not activated or a BP component like “Messages”, “Friends” or “xProfile fields” are not activated). To go further, we invite you to ask for more help from the plugin authors and post your request on their support forums: https://wordpress.org/support/plugin/bp-birthday-greetings/
Finally, please also check all the WordPress prerequisites here: https://wordpress.org/about/requirements/.
For better performance (and better security) we obviously recommend the version 7.2 for PHP. Nevertheless, if this is not possible for you (server type, incompatibilities with plugins, etc.) we recommend at least version 5.6.X.
We hope this will help.
Regards,