Branding extension didn’t work at first since, as MainWP failed to add Child Site without Unique ID and HTTP Add method didn’t work for me.
However, I took a different approach to this problem and did the following steps:
First, I retrieved the Unique ID on Child Site by visiting the following link on ChildSite domain.com/wp-admin/options.php
Added the Unique ID on MainWP Dashboard to Add the Child Site with retrieved ID
Re-applied my branding settings using Branding Extension on the Child Site from MainWP and everything started working fine
so yeah if someone is facing the same issue as mine, maybe this method could work!