Refactor: Replace default_scheme_variant with is_mono flag for monochrome setting

Author: lainsce

src/main.vala

@@ -44,7 +44,7 @@ namespace Colorway {
             Gdk.RGBA accent_color = { 0 };
             accent_color.parse ("#75DEC2");
             default_accent_color = He.from_gdk_rgba (accent_color);
-            default_scheme_variant = He.SchemeVariant.MONOCHROME;
+            is_mono = true;
 
             resource_base_path = "/io/github/lainsce/Colorway";