[Themes] Ship Fusion style + scheme backgrounds for the image themes

Address PR review:
- Add theme.cfg ([Style] Name = Fusion, ColorScheme = System) to Fabric,
  Leather, Plasma and VelvetMarble so their curated palettes actually apply.
  Without it the native style paints button chrome from the OS (windowsvista
  on Windows has no dark mode), making the dark palettes' light ButtonText
  unreadable on light buttons.
- Leverage the scheme-variant asset resolution: register home-dark.png and
  home-light.png in resources and add them to the qrc so the built-in home
  background also flips with the palette instead of staying static.
- Fix Leather [Palette.Inactive] Accent, which copied the Active Highlight
  color instead of the theme accent (dark #4a5f8f -> #c9995a, light
  #34508c -> #a5712f) in both palette files.

cockatrice/cockatrice.qrc

@@ -63,6 +63,8 @@
         <file>resources/icons/mana/W.svg</file>
 
         <file>resources/backgrounds/home.png</file>
+        <file>resources/backgrounds/home-dark.png</file>
+        <file>resources/backgrounds/home-light.png</file>
         <file>resources/backgrounds/card_triplet.svg</file>
         <file>resources/backgrounds/placeholder_printing_selector.svg</file>
 

cockatrice/themes/Fabric/theme.cfg

@@ -0,0 +1,5 @@
+[Appearance]
+ColorScheme = System
+
+[Style]
+Name = Fusion

cockatrice/themes/Leather/palette-default-dark.toml

@@ -66,7 +66,7 @@ AlternateBase        = #211d19
 ToolTipBase          = #37302a
 ToolTipText          = #f5f1ea
 PlaceholderText      = #6eeee9e2
-Accent               = #4a5f8f
+Accent               = #c9995a
 
 [AppColors]
 AccentStrong         = #b7823b

cockatrice/themes/Leather/palette-default-light.toml

@@ -66,7 +66,7 @@ AlternateBase        = #f5ebd7
 ToolTipBase          = #fff7e8
 ToolTipText          = #1f1710
 PlaceholderText      = #7a2a2114
-Accent               = #34508c
+Accent               = #a5712f
 
 [AppColors]
 AccentStrong         = #b7823b

cockatrice/themes/Leather/theme.cfg

@@ -0,0 +1,5 @@
+[Appearance]
+ColorScheme = System
+
+[Style]
+Name = Fusion

cockatrice/themes/Plasma/theme.cfg

@@ -0,0 +1,5 @@
+[Appearance]
+ColorScheme = System
+
+[Style]
+Name = Fusion

cockatrice/themes/VelvetMarble/theme.cfg

@@ -0,0 +1,5 @@
+[Appearance]
+ColorScheme = System
+
+[Style]
+Name = Fusion