Automatic face re-application with frame-set-background-mode

Hello,

I’m experimenting with themes on Emacs (using master branch) and
noticed some behavior related to `frame-set-background-mode` that I’d
like to clarify. 

I created a minimal working example theme as follows:

```
;; -*- lexical-binding: t; -*-
(deftheme mve-theme "Minimal MVE")

(custom-theme-set-faces
 'default
 `(default
   ((((background light)) :background "white" :foreground "black")
    (((background dark))  :background "black" :foreground "white")))
 `(mode-line
   ((((background light)) :background "yellow" :foreground "black")
    (((background dark))  :background "red" :foreground "white"))))

(provide-theme 'mve-theme)
```

Steps I performed:

Run `emacs -Q`
Load the theme with `M-x load-theme mve-theme`.
Call `(setopt frame-background-mode 'dark)`.

Notable observations:

* The fringe (unspecified in this theme) changes to black.
* The mode-line stays yellow.
* The default face does not change. This is true even with the
`default` theme.
* Syntax highlighting colors in emacs-lisp-mode update according to
background-mode.

It seems that some faces do get re-applied after a `background-mode`
switch, while others do not. Even when they do, they do not honor my
`custom-theme-set-faces` settings.

I’m trying to understand:

What conditions determine which faces are automatically recalculated
when `frame-set-background-mode` is called?
Is this behavior hardcoded for built-in faces only?
Are there plans or best practices for making theme faces reactive to
background-mode changes without manually reapplying the theme?

Thank you for any clarification or guidance.

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