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/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 {