Render admin pages (forms) alongwith their admin theme from within a custom theme
Theme nesting is not implemented in Drupal. Too much risk of CSS conflicts, and HTML and CSS does not provide any way of isolation except for iframe. There is, quite literally, no way to write an universal solution. So you may:


Implement resets as you need. Probably hard, but still infinitely easier than any attempt to make solution universal and foolproof.
Trim the admin theme to remove footer, sidebars etc, and open admin pages in iframe
Prepare your own theme in a way that will render admin pages in a convenient way, "borrowing" code from admin theme you like (probably it's GPL, so you are allowed to do it).