Blocks added to mega menu not visible on switching to another theme

@Dharna Tyagi
Are you getting any error in console?
If yes then share screenshot.
The issue may be due to JS error in you new theme so console error code may be help to understand issue.

@Dharna Tyagi
After review again, I got the issue that you need to assign block in your new theme.

To resolve this issue , need to write script to update configuration data for a Mega Menu block once after the theme gets switched.