feat(theme): add --header-background-color CSS variable for image-free headers

Fix typo (backgroun-color → background-color) in .container-header that
silently broke the color fallback.  Split background shorthand into
discrete background-color / background-image properties so the color
renders when --header-background-image is set to none.

Add --header-background-color to all four theme files (dark.standard,
light.standard, dark.custom, light.custom).  Regenerate minified CSS.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/templates/dark.custom.css

@@ -210,6 +210,7 @@ color-scheme: dark;
 
 
 /* ===== HEADER BACKGROUND ===== */
+--header-background-color: #1a1f2b;
 --header-background-image: url('../../../../../../media/templates/site/mokoonyx/images/bg.svg');
 --header-background-attachment: fixed;
 --header-background-repeat: repeat;

src/templates/light.custom.css

@@ -209,6 +209,7 @@ color-scheme: light;
 
 
 /* ===== HEADER BACKGROUND ===== */
+--header-background-color: #adadad;
 --header-background-image: url('../../../../../../media/templates/site/mokoonyx/images/bg.svg');
 --header-background-attachment: fixed;
 --header-background-repeat: repeat;