refactor(demos): enable theming legacy support in demos

Closes #5362

Author: simeonoff

src/styles/igniteui-theme.scss

@@ -17,7 +17,7 @@
     color: $igx-foreground-color;
 
     @include scrollbar-love();
-    @include igx-theme($default-palette, $legacy-support: false);
+    @include igx-theme($default-palette);
 
     .nav-header {
         @include nav-logo('../assets/images/rsrcs/igniteui-logo-light-bg', $igx-background-color);