During the course of the upgrade, it is found that after the upgrade drupal 10 consistently fails to recognize bootstrap theme, resulting in a site with plain html and css, this post will help you resolve this issue.
Step-1: Install using composer
To begin with you will need to install the theme using composer, this will place your theme inside the $drupal-root/theme/contrib folder, for instance run composer require 'drupal/bootstrap:^3.29' will download the drupal bootstrap 3.29 theme and place it in folder $drupal-root/theme/contrib/bootstrap.
Step-2: Remove original theme
Then you will need to remove the original folder located at the $drupal-root/theme for this instance, the original bootstrap base theme is located at $drupal-root/theme/bootstrap, you will need to remove it.
Step-3: Clear Cache
Lastly, we will need to let drupal correctly identify the location of the new composer installed theme, simply clear the cache using the admin panel or run drush cr in terminal.