Is it possible to inherit theme files?
If I am not mistaken, base themes do exactly what you describe.


For Drupal 7: https://www.drupal.org/docs/7/theming/creating-a-sub-theme
For Drupal 8: https://www.drupal.org/docs/8/theming-drupal-8/creating-a-drupal-8-sub-theme-or-sub-theme-of-sub-theme


In your case, you can use 'Theme A' as the base theme, and specify that 'Theme B' uses 'Theme A' as the base in the .info file. You can then override the CSS and other theme features as you wish.