diff --git a/src/calibre/gui2/tweak_book/preferences.py b/src/calibre/gui2/tweak_book/preferences.py
index dc4c022117..cac49bc710 100644
--- a/src/calibre/gui2/tweak_book/preferences.py
+++ b/src/calibre/gui2/tweak_book/preferences.py
@@ -407,7 +407,9 @@ class PreviewSettings(BasicSettings):  # {{{
             ans.setObjectName(name)
             return ans
 
-        b('unset', _('No change'), _('Use the colors from the book styles, defaulting to black-on-white'))
+        b('unset', _('No change'), _('Use the colors from the book styles, defaulting to black-on-white.'
+                                     ' Note that in dark mode, you must set all three colors to "No change"'
+                                     ' otherwise the book is rendered with dark colors.'))
         b('auto', _('Theme based'), _('When using a dark theme force dark colors, otherwise same as "No change"'))
         b('manual', _('Custom'), _('Choose a custom color'))
 
diff --git a/src/calibre/gui2/tweak_book/preview.py b/src/calibre/gui2/tweak_book/preview.py
index fa7d1b1015..8cd226ef5a 100644
--- a/src/calibre/gui2/tweak_book/preview.py
+++ b/src/calibre/gui2/tweak_book/preview.py
@@ -277,7 +277,9 @@ def create_dark_mode_script():
         function apply_css() {
             var css = '';
             if (settings.link) css += 'html > body :link, html > body :link * { color: ' + settings.link + ' !important; }';
-            if (settings.is_dark_theme) { css = ':root { color-scheme: dark; }' + css; }
+            var using_custom_colors = false;
+            if (settings.bg || settings.fg || settings.link) using_custom_colors = true;
+            if (settings.is_dark_theme && using_custom_colors) { css = ':root { color-scheme: dark; }' + css; }
             var style = document.createElement('style');
             style.textContent = css;
             document.documentElement.appendChild(style);