fixes for veilid 0.4.0

lib/theme/models/scale_scheme.dart

@@ -145,3 +145,33 @@ class ScaleConfig extends ThemeExtension<ScaleConfig> {
             lerpDouble(borderRadiusScale, other.borderRadiusScale, t) ?? 1);
   }
 }
+
+class ScaleTheme extends ThemeExtension<ScaleTheme> {
+  ScaleTheme({
+    required this.scheme,
+    required this.config,
+  });
+
+  final ScaleScheme scheme;
+  final ScaleConfig config;
+
+  @override
+  ScaleTheme copyWith({
+    ScaleScheme? scheme,
+    ScaleConfig? config,
+  }) =>
+      ScaleTheme(
+        scheme: scheme ?? this.scheme,
+        config: config ?? this.config,
+      );
+
+  @override
+  ScaleTheme lerp(ScaleTheme? other, double t) {
+    if (other is! ScaleTheme) {
+      return this;
+    }
+    return ScaleTheme(
+        scheme: scheme.lerp(other.scheme, t),
+        config: config.lerp(other.config, t));
+  }
+}