fix(platform): 外观主题 - 保证经典黑白主题不可被修改或删除

backend/internal/app/appearance_button_fill_test.go

@@ -47,8 +47,12 @@ func TestAppearanceButtonFillRoundTripAndValidation(t *testing.T) {
 		})
 	}
 	input.SkinThemes[0].Tokens.Buttons.Light.Start = "#000000"
-	if _, err := svc.UpdateAppearance(admin, input); err == nil {
-		t.Fatal("classic fill mutation accepted")
+	overwritten, err := svc.UpdateAppearance(admin, input)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if overwritten.SkinThemes[0].Tokens.Buttons != defaultClassicAppearanceSkin().Tokens.Buttons {
+		t.Fatal("classic fill mutation was stored")
 	}
 }