diff --git a/src/framework/ui/internal/themeconverter.cpp b/src/framework/ui/internal/themeconverter.cpp
index e50e0a6ed445b..a3d515a731a8e 100644
--- a/src/framework/ui/internal/themeconverter.cpp
+++ b/src/framework/ui/internal/themeconverter.cpp
@@ -138,7 +138,10 @@ ThemeInfo ThemeConverter::fromMap(const QVariantMap& map)
     theme.title = map[TITLE_KEY].toString().toStdString();
 
     for (const QString& key : map.keys()) {
-        theme.values[themeStyleKeyFromString(key)] = map[key];
+        ThemeStyleKey styleKey = themeStyleKeyFromString(key);
+        if (styleKey != UNKNOWN) {
+            theme.values[styleKey] = map[key];
+        }
     }
 
     return theme;
diff --git a/src/framework/ui/internal/uiconfiguration.cpp b/src/framework/ui/internal/uiconfiguration.cpp
index 3f3d38f8ddc44..d6c86b54da670 100644
--- a/src/framework/ui/internal/uiconfiguration.cpp
+++ b/src/framework/ui/internal/uiconfiguration.cpp
@@ -216,7 +216,11 @@ void UiConfiguration::updateThemes()
 
         bool isModified = it != modifiedThemes.end();
         if (isModified) {
-            theme = *it;
+            for (auto key : it->values.keys()) {
+                if (key != UNKNOWN) {
+                    theme.values[key] = it->values[key];
+                }
+            }
         }
     }
 }
diff --git a/src/framework/ui/tests/CMakeLists.txt b/src/framework/ui/tests/CMakeLists.txt
index 35647dc1038bb..24cb02025e135 100644
--- a/src/framework/ui/tests/CMakeLists.txt
+++ b/src/framework/ui/tests/CMakeLists.txt
@@ -27,6 +27,7 @@ set(MODULE_TEST_SRC
     ${CMAKE_CURRENT_LIST_DIR}/mocks/mainwindowmock.h
 
     ${CMAKE_CURRENT_LIST_DIR}/navigationcontroller_tests.cpp
+    ${CMAKE_CURRENT_LIST_DIR}/themeconverter_tests.cpp
     )
 
 set(MODULE_TEST_LINK
diff --git a/src/framework/ui/tests/themeconverter_tests.cpp b/src/framework/ui/tests/themeconverter_tests.cpp
new file mode 100644
index 0000000000000..d5eebd2b8c3e6
--- /dev/null
+++ b/src/framework/ui/tests/themeconverter_tests.cpp
@@ -0,0 +1,164 @@
+/*
+ * SPDX-License-Identifier: GPL-3.0-only
+ * MuseScore-CLA-applies
+ *
+ * MuseScore
+ * Music Composition & Notation
+ *
+ * Copyright (C) 2021 MuseScore Limited and others
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 3 as
+ * published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+#include <gtest/gtest.h>
+
+#include <QColor>
+#include <QVariantMap>
+
+#include "ui/internal/themeconverter.h"
+#include "ui/uitypes.h"
+
+using namespace muse;
+using namespace muse::ui;
+
+class Ui_ThemeConverterTests : public ::testing::Test
+{
+};
+
+TEST_F(Ui_ThemeConverterTests, FromMap_SkipsUnknownKeys)
+{
+    //! [GIVEN] A map containing metadata keys, a known style key, and an unknown key
+    QVariantMap map;
+    map["codeKey"] = "dark";
+    map["title"] = "Dark";
+    map["backgroundPrimaryColor"] = QColor("#2D2D30");
+    map["someRandomKey"] = QColor("#FF0000");
+
+    //! [WHEN] Deserializing via fromMap
+    ThemeInfo theme = ThemeConverter::fromMap(map);
+
+    //! [THEN] Only the known style key is in values; UNKNOWN is not present
+    EXPECT_EQ(theme.codeKey, "dark");
+    EXPECT_EQ(theme.title, "Dark");
+    EXPECT_TRUE(theme.values.contains(BACKGROUND_PRIMARY_COLOR));
+    EXPECT_FALSE(theme.values.contains(UNKNOWN));
+    EXPECT_EQ(theme.values.size(), 1);
+}
+
+TEST_F(Ui_ThemeConverterTests, FromMap_DoesNotPopulateExtra)
+{
+    //! [GIVEN] A map with known style keys
+    QVariantMap map;
+    map["codeKey"] = "dark";
+    map["title"] = "Dark";
+    map["accentColor"] = QColor("#2093FE");
+    map["fontPrimaryColor"] = QColor("#EBEBEB");
+
+    //! [WHEN] Deserializing via fromMap
+    ThemeInfo theme = ThemeConverter::fromMap(map);
+
+    //! [THEN] The extra map remains empty
+    EXPECT_TRUE(theme.extra.isEmpty());
+}
+
+TEST_F(Ui_ThemeConverterTests, ToMap_DoesNotSerializeExtra)
+{
+    //! [GIVEN] A theme with both values and extra entries
+    ThemeInfo theme;
+    theme.codeKey = "dark";
+    theme.title = "Dark";
+    theme.values[ACCENT_COLOR] = QColor("#2093FE");
+    theme.extra["custom_highlight"] = QColor("#FF0000");
+    theme.extra["custom_shadow"] = QColor("#000000");
+
+    //! [WHEN] Serializing via toMap
+    QVariantMap map = ThemeConverter::toMap(theme);
+
+    //! [THEN] Known keys are present but extra keys are not serialized
+    EXPECT_TRUE(map.contains("codeKey"));
+    EXPECT_TRUE(map.contains("accentColor"));
+    EXPECT_FALSE(map.contains("custom_highlight"));
+    EXPECT_FALSE(map.contains("custom_shadow"));
+}
+
+TEST_F(Ui_ThemeConverterTests, ToMap_FromMap_RoundTrip_PreservesKnownValues)
+{
+    //! [GIVEN] A theme with color, width, and opacity values
+    ThemeInfo original;
+    original.codeKey = "dark";
+    original.title = "Dark";
+    original.values[BACKGROUND_PRIMARY_COLOR] = QColor("#2D2D30");
+    original.values[ACCENT_COLOR] = QColor("#2093FE");
+    original.values[FONT_PRIMARY_COLOR] = QColor("#EBEBEB");
+    original.values[BORDER_WIDTH] = 1.0;
+    original.values[ACCENT_OPACITY_NORMAL] = 0.5;
+
+    //! [WHEN] Round-tripping through toMap then fromMap
+    QVariantMap map = ThemeConverter::toMap(original);
+    ThemeInfo roundTripped = ThemeConverter::fromMap(map);
+
+    //! [THEN] codeKey and all values survive the round-trip
+    EXPECT_EQ(roundTripped.codeKey, original.codeKey);
+
+    for (auto key : original.values.keys()) {
+        ASSERT_TRUE(roundTripped.values.contains(key))
+            << "Missing key after round-trip: " << static_cast<int>(key);
+        EXPECT_EQ(roundTripped.values[key], original.values[key])
+            << "Value mismatch for key: " << static_cast<int>(key);
+    }
+}
+
+TEST_F(Ui_ThemeConverterTests, MergeScenario_ExtrasPreservedAfterRoundTrip)
+{
+    //! CASE Simulates the full updateThemes() data flow:
+    //!      writeThemes (toMap) -> readThemes (fromMap) -> merge into cfg-backed theme
+
+    //! [GIVEN] A cfg-backed theme with values AND extras (as makeStandardTheme produces)
+    ThemeInfo baseTheme;
+    baseTheme.codeKey = "dark";
+    baseTheme.title = "Dark";
+    baseTheme.values[BACKGROUND_PRIMARY_COLOR] = QColor("#2D2D30");
+    baseTheme.values[ACCENT_COLOR] = QColor("#2093FE");
+    baseTheme.values[FONT_PRIMARY_COLOR] = QColor("#EBEBEB");
+    baseTheme.values[BORDER_WIDTH] = 0.0;
+    baseTheme.extra["accent_color"] = QColor("#2093FE");
+    baseTheme.extra["background_primary_color"] = QColor("#2D2D30");
+    baseTheme.extra["custom_plugin_color"] = QColor("#AABBCC");
+
+    //! [GIVEN] The theme is serialized to settings (extras are dropped by toMap)
+    QVariantMap serialized = ThemeConverter::toMap(baseTheme);
+
+    //! [GIVEN] The user customizes accent color
+    serialized["accentColor"] = QColor("#FF5500");
+
+    //! [WHEN] The theme is deserialized from settings and merged back
+    //!        (replicating the updateThemes loop from uiconfiguration.cpp)
+    ThemeInfo fromSettings = ThemeConverter::fromMap(serialized);
+    for (auto key : fromSettings.values.keys()) {
+        if (key != UNKNOWN) {
+            baseTheme.values[key] = fromSettings.values[key];
+        }
+    }
+
+    //! [THEN] The customized value is applied
+    EXPECT_EQ(baseTheme.values[ACCENT_COLOR].value<QColor>(), QColor("#FF5500"));
+
+    //! [THEN] Other values are preserved
+    EXPECT_EQ(baseTheme.values[BACKGROUND_PRIMARY_COLOR].value<QColor>(), QColor("#2D2D30"));
+    EXPECT_EQ(baseTheme.values[FONT_PRIMARY_COLOR].value<QColor>(), QColor("#EBEBEB"));
+
+    //! [THEN] Extras are fully intact
+    EXPECT_EQ(baseTheme.extra.size(), 3);
+    EXPECT_EQ(baseTheme.extra["accent_color"].value<QColor>(), QColor("#2093FE"));
+    EXPECT_EQ(baseTheme.extra["background_primary_color"].value<QColor>(), QColor("#2D2D30"));
+    EXPECT_EQ(baseTheme.extra["custom_plugin_color"].value<QColor>(), QColor("#AABBCC"));
+}