Pure CSS ePub theme detection (White / Sepia / Night) in iBooks (Mac OS and iOS included)

theme-detect.css

/*
Pure CSS ePub theme detection (White / Sepia / Night) in iBooks (Mac OS and iOS included)
Version 1.1
Note that iBooks actually uses internal JavaScript to set the "__ibooks_internal_theme" attribute on :root (html) element
every time the theme is changed, but this happens independently of whether your epub html markup is scripted or not.

Discovered and tested in iBooks on Mac, iPhone and iPad by
https://twitter.com/adaptivegarage
*/

/* For White Theme */
/* ... write your primary CSS here ... */

/* For Sepia Theme */
:root[__ibooks_internal_theme*="Sepia"] /* elements */ {
    /* ... override your CSS here ... */
}

/* For Night Theme PLEASE READ COMMENTS BELOW */
:root[__ibooks_internal_theme*="Night"] /* elements */ {
    /* ... override background-color: */
    background: -webkit-linear-gradient(/* color */, /* (the same) color */);

    /* ... override text color: */
    -webkit-text-fill-color: /* color */;

    /* ... override rest of your CSS here ... */
}

/*
PLEASE READ CAREFULLY:
iBooks adjusts HTML and CSS markup behind the scenes in many ways and when the Night theme is selected (not White or Sepia),
these CSS styles (among others) are forced:

:root {
    background-color: black !important;
}

:root * {
    background-color: transparent !important;
    color: #bebebe !important;
}

If you need to override background or text color of *any* element in the Night theme, use the CSS above, but please WISELY
and only when absolutely necessary.
I believe that abusing this method will cause it banned by Apple in future iBooks releases.
*/

@marketmann @robinwhittleton @poisontofu @ColinLondon @adaptivegarage

With iOS 16 there are potentially 12 background colours (themes).
But in iOS 16, Books does not seem to set the "__ibooks_internal_theme" attribute on :root (html) element at all!

Attributes on :root (macOS Monterey 12.5.1)
0: xmlns, (as user entered in source)
1: xmlns:epub , (as user entered in source)
2: __ibooks_internal_theme
3: __ibooks_internal_level , 1
4: __ibooks_writing_mode , horizontal-tb

Attributes on :root (iOS 16)
0: xmlns , (as user entered in source)
1: xmlns:epub , (as user entered in source)
2: __ibooks_content_is_japanese, false
3: __ibooks_content_is_chinese , false
4: __ibooks_reading_mode_paged
5: __ibooks_internal_theme , always returns undefined!
6: __ibooks_internal_level
7: __ibooks_writing_mode, horizontal-tb