Steps to reproduce the issue

Try to use own buttons and button groups in modal

Expected result

See the buttons in correct Boostrap style

Actual result

Button groups and buttons have invalid style due to:

.modal .btn { ... }

It's bad idea to change style for ALL buttons in modal.

Probably, we can force this style for .modal-footer only:

.modal .modal-footer .btn {..}

But note that even in this case we can't have colored buttons (btn-success etc.) due to current CSS, TDB.