Hi Shubha, thank you for contacting us!
The problem you are seeing appears to be caused by some class names for HTML elements that are shared between your theme and the Post and Page Builder. The quickest way to fix this particular problem is to use the Custom CSS option in the Customizer to address the styling inconsistencies. Here is some code I wrote that should address the problems you are seeing:
@media (max-width: 767px) {
#navbar .row > div {
width: 100%;
}
#navbar .navbar-collapse.in {
height: auto;
}
}
Please let us know if the fix works or if you have any other questions for us!