Add the Fusion Black and Fusion White themes

Fusion Black pairs near-black chrome with a paper-white canvas, so the
drawing is the only bright thing on screen. That breaks the assumption
in theme_canvas_background that the canvas follows the chrome, hence
the explicit lookup: falling through to the palette would paint model
space black and undo the point of the theme.

Both keep a blue accent rather than going fully greyscale. Selection
highlight defaults to the theme primary, and a white primary would be
invisible against a white canvas.

These are Theme::Custom values and so are absent from iced's
Theme::ALL. all_themes() is the list to enumerate from now; the picker
in the options dialog uses it, and parse_theme_name resolves both by
name.

src/app/config.rs

@@ -176,11 +176,20 @@ impl UiThemePalette {
     }
 }
 
-pub fn builtin_theme(name: &str) -> Option<iced::Theme> {
+/// Every theme the user may pick: iced's built-ins followed by the Fusion
+/// pair. The Fusion themes are `Theme::Custom`, so they are not in
+/// `iced::Theme::ALL`; anything enumerating themes for display or for test
+/// coverage must use this instead, or they silently vanish from the list.
+pub fn all_themes() -> Vec<iced::Theme> {
     iced::Theme::ALL
         .iter()
-        .find(|theme| theme.to_string() == name)
         .cloned()
+        .chain(crate::ui::style::fusion_theme::fusion_themes())
+        .collect()
+}
+
+pub fn builtin_theme(name: &str) -> Option<iced::Theme> {
+    all_themes().into_iter().find(|theme| theme.to_string() == name)
 }
 
 fn color_to_rgb(color: iced::Color) -> [u8; 3] {
@@ -376,7 +385,14 @@ pub fn theme_canvas_background(theme: &iced::Theme) -> [u8; 3] {
         iced::Theme::Nightfly => [1, 22, 39],
         iced::Theme::Oxocarbon => [22, 22, 22],
         iced::Theme::Ferra => [43, 41, 46],
-        _ => color_to_rgb(theme.palette().background.base.color),
+        // The Fusion pair deliberately breaks the "canvas follows chrome"
+        // rule: black chrome is paired with a white canvas, so falling
+        // through to the palette below would paint model space black and
+        // undo the whole point of the theme.
+        other => match crate::ui::style::fusion_theme::fusion_canvas(other) {
+            Some(rgb) => rgb,
+            None => color_to_rgb(other.palette().background.base.color),
+        },
     }
 }
 
@@ -407,6 +423,8 @@ pub fn parse_theme_name(s: &str) -> Option<iced::Theme> {
         "NIGHTFLY" => Some(iced::Theme::Nightfly),
         "OXOCARBON" => Some(iced::Theme::Oxocarbon),
         "FERRA" => Some(iced::Theme::Ferra),
+        "FUSIONBLACK" => Some(crate::ui::style::fusion_theme::fusion_black()),
+        "FUSIONWHITE" => Some(crate::ui::style::fusion_theme::fusion_white()),
         _ => None,
     }
 }

src/ui/style/fusion_theme.rs

@@ -0,0 +1,140 @@
+//! The Fusion theme pair: black chrome around a white canvas, and its
+//! inverse.
+//!
+//! "Mixed black/white" is about the split, not about draining every colour
+//! out of the UI. Ribbon, docks, status bar and command line sit at
+//! near-black while model space stays paper-white, so the drawing is the
+//! only bright thing on screen. One accent survives that split: selection
+//! highlight defaults to the theme primary
+//! (`ModelSpaceThemeConfig::selection_highlight_color` = 0), and a white
+//! primary would be invisible against a white canvas. The accent is
+//! therefore a saturated blue that reads against both halves.
+//!
+//! These are `iced::Theme::Custom` values, so they are absent from
+//! `iced::Theme::ALL`. Anything enumerating themes must go through
+//! [`crate::app::config::all_themes`] instead.
+
+use iced::theme::palette::Seed;
+use iced::{Color, Theme};
+
+/// Display name of the black-chrome theme. Persisted verbatim in
+/// `settings.json` as `theme.name`, so changing it orphans a user's choice.
+pub const FUSION_BLACK: &str = "Fusion Black";
+
+/// Display name of the light-chrome theme.
+pub const FUSION_WHITE: &str = "Fusion White";
+
+/// Model-space canvas for [`FUSION_BLACK`] — paper white against the
+/// near-black chrome. Deliberately not pure #FFFFFF: a fraction below white
+/// keeps the paper-space sheet distinguishable from the model canvas.
+pub const FUSION_BLACK_CANVAS: [u8; 3] = [250, 250, 250];
+
+/// Model-space canvas for [`FUSION_WHITE`].
+pub const FUSION_WHITE_CANVAS: [u8; 3] = [255, 255, 255];
+
+fn rgb(hex: u32) -> Color {
+    Color::from_rgb8(
+        ((hex >> 16) & 0xFF) as u8,
+        ((hex >> 8) & 0xFF) as u8,
+        (hex & 0xFF) as u8,
+    )
+}
+
+/// Near-black chrome, white text, blue accent.
+pub fn fusion_black() -> Theme {
+    Theme::custom(
+        FUSION_BLACK,
+        Seed {
+            background: rgb(0x1A1A1A),
+            text: rgb(0xF2F2F2),
+            primary: rgb(0x0696D7),
+            success: rgb(0x4CAF50),
+            warning: rgb(0xFFB300),
+            danger: rgb(0xE53935),
+        },
+    )
+}
+
+/// The inverse: light chrome, near-black text. The accent darkens so it
+/// keeps its contrast against a light surface.
+pub fn fusion_white() -> Theme {
+    Theme::custom(
+        FUSION_WHITE,
+        Seed {
+            background: rgb(0xFAFAFA),
+            text: rgb(0x1A1A1A),
+            primary: rgb(0x0277BD),
+            success: rgb(0x2E7D32),
+            warning: rgb(0xE65100),
+            danger: rgb(0xC62828),
+        },
+    )
+}
+
+/// Both Fusion themes, in the order the theme picker should show them.
+pub fn fusion_themes() -> Vec<Theme> {
+    vec![fusion_black(), fusion_white()]
+}
+
+/// The canvas colour a Fusion theme asks for, or `None` for any other theme.
+/// Keyed by name because `Theme::Custom` carries no discriminant to match on.
+pub fn fusion_canvas(theme: &Theme) -> Option<[u8; 3]> {
+    match theme.to_string().as_str() {
+        FUSION_BLACK => Some(FUSION_BLACK_CANVAS),
+        FUSION_WHITE => Some(FUSION_WHITE_CANVAS),
+        _ => None,
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::ui::style::common::wcag_contrast;
+
+    /// The same bar `theme_accessibility_tests` holds the built-in themes to.
+    /// These themes are hand-written rather than generated, so nothing else
+    /// would catch a palette typo that drops text below AA.
+    #[test]
+    fn fusion_themes_meet_wcag_aa_on_every_surface() {
+        for theme in fusion_themes() {
+            let p = theme.palette();
+            for (name, pair) in [
+                ("base", p.background.base),
+                ("weak", p.background.weak),
+                ("strong", p.background.strong),
+                ("weakest", p.background.weakest),
+            ] {
+                let contrast = wcag_contrast(pair.text, pair.color);
+                assert!(
+                    contrast >= 4.5,
+                    "{theme} {name} surface text is {contrast:.2}:1, below AA"
+                );
+            }
+        }
+    }
+
+    /// The accent exists so selection stays visible on the white canvas. If
+    /// it ever drifts toward white this catches it.
+    #[test]
+    fn fusion_accent_is_visible_on_the_canvas() {
+        for (theme, canvas) in [
+            (fusion_black(), FUSION_BLACK_CANVAS),
+            (fusion_white(), FUSION_WHITE_CANVAS),
+        ] {
+            let canvas = Color::from_rgb8(canvas[0], canvas[1], canvas[2]);
+            let contrast = wcag_contrast(theme.palette().primary.base.color, canvas);
+            assert!(
+                contrast >= 3.0,
+                "{theme} accent is {contrast:.2}:1 on its canvas, too faint \
+                 to show a selection"
+            );
+        }
+    }
+
+    #[test]
+    fn fusion_canvas_is_keyed_by_name() {
+        assert_eq!(fusion_canvas(&fusion_black()), Some(FUSION_BLACK_CANVAS));
+        assert_eq!(fusion_canvas(&fusion_white()), Some(FUSION_WHITE_CANVAS));
+        assert_eq!(fusion_canvas(&Theme::Dark), None);
+    }
+}

src/ui/style/mod.rs

@@ -11,6 +11,7 @@ pub mod anno_object_scale;
 pub mod style_manager;
 pub mod common;
 pub mod form;
+pub mod fusion_theme;
 
 #[cfg(test)]
 mod common_tests {

src/ui/window/options.rs

@@ -179,7 +179,7 @@ pub fn view_window<'a>(
         .copied()
         .find(|candidate| *candidate == default_save_format);
 
-    let theme_options = Theme::ALL
+    let theme_options = crate::app::config::all_themes()
         .iter()
         .map(ToString::to_string)
         .chain(std::iter::once("Custom".to_string()))