chore: fix sheet header color to match new style

Author: amandaye0h

app/component-library/components/Sheet/SheetHeader/SheetHeader.styles.ts

@@ -23,7 +23,7 @@ const styleSheet = (params: { theme: Theme }) => {
       margin: 16,
       backgroundColor:
         themeAppearance === AppThemeKey.dark
-          ? colors.background.section
+          ? colors.background.alternative
           : colors.background.default,
       height: 32,
     },