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/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()))