Support for overriding faces with theme colors

I've just started using these themes and I really like them! Thanks for the great work.

I am spending a lot of time using emacsclient in the terminal, and overall your themes are fantastic, colors are all great. In a few places, they're using features that do not display in the terminal, like :box

I would like to be able to override these in my init. For example:

(use-package modus-vivendi-theme
   :init
   (setq modus-vivendi-theme-visible-fringe t
         modus-vivendi-theme-3d-modeline t)
   :config
   (defun customize-modus-vivendi ()
     "Customize modus-vivendi theme"
     (if (member 'modus-vivendi custom-enabled-themes)
         (custom-theme-set-faces
          'modus-vivendi
          `(magit-branch-current ((t (:foreground "#00baf4" :background "#181a20" :box t))))
          `(magit-branch-remote-head ((t (:foreground "#b6a0ff" :background "#181a20" :box t))))
          )))
   (add-hook 'after-load-theme-hook 'customize-modus-vivendi)
   (load-theme 'modus-vivendi t))

(Here I have just added a background modifier in addition to the box, so that the box is still used on graphical displays, but it may be possible to include two definitions, one which explicitly tests the support for :box attribute, and then a fallback)

But I need to use the exact colors here. The zenburn theme provides a zenburn-with-color-variables function to allow overriders to access the zenburn color names. Maybe you could do something similar here? Or perhaps there is some other standard mechanism to allow end-user refinement of individual faces.