customizing faces/themes

[Andrea Crotti <andrea.crotti.0@gmail.com>]

I use 2 or 3 themes, normally dark and I noticed a "problem".

I set this faces with the customization framework, and those faces are
not defined for the theme X.
But they are defined for the them Y, so when I switch to Y they are set
to another value, and when I go back to X, they don't revert to my
customization.

So I guess the only way is to add some faces to one theme I like,
correct?

But what if I want to use the same faces with more than one theme?
What's the best way to solve this?

 '(diff-added ((t (:inherit diff-changed :background "grey4" :foreground "Yellow"))))
 '(diff-changed ((nil (:foreground "medium purple"))))
 '(diff-header ((((class color) (min-colors 88) (background dark)) (:background "grey22" :foreground "red" :height 1.2))))
 '(diff-indicator-removed ((t (:inherit diff-removed))))
 '(diff-removed ((t (:background "grey5" :foreground "medium spring green"))))