fix(layout): set header background color on host element (#1823)

Author: yggg

src/framework/theme/components/layout/_layout.component.theme.scss

@@ -177,14 +177,14 @@
   }
 
   nb-layout-header {
+    background-color: nb-theme(header-background-color);
     color: nb-theme(header-text-color);
     font-family: nb-theme(header-text-font-family);
     font-size: nb-theme(header-text-font-size);
     font-weight: nb-theme(header-text-font-weight);
     line-height: nb-theme(header-text-line-height);
 
     nav {
-      background: nb-theme(header-background-color);
       color: nb-theme(header-text-color);
       box-shadow: nb-theme(header-shadow);
       height: nb-theme(header-height);