diff --git a/lib/io/include/io/internal/filesystem.hpp b/lib/io/include/io/internal/filesystem.hpp
index dd05ab70f..ebd8a3e2f 100644
--- a/lib/io/include/io/internal/filesystem.hpp
+++ b/lib/io/include/io/internal/filesystem.hpp
@@ -40,6 +40,7 @@ namespace VTX::IO::Internal::Filesystem
 	inline const FilePath getLibrariesDir() { return getExecutableDir() / "libraries"; }
 	inline const FilePath getRepresentationsLibraryDir() { return getLibrariesDir() / "representations"; }
 	inline const FilePath getRenderEffectPresetsLibraryDir() { return getLibrariesDir() / "render_effects"; }
+	inline const FilePath getThemesLibraryDir() { return getLibrariesDir() / "themes"; }
 	inline const FilePath getInternalDataDir() { return getExecutableDir() / "data"; }
 	inline const FilePath getResidueDataDir() { return getInternalDataDir() / "residue_data"; }
 
diff --git a/lib/ui/asset/qt/resources/resources_ui.qrc b/lib/ui/asset/qt/resources/resources_ui.qrc
index f56b0efba..0dc4c7d75 100644
--- a/lib/ui/asset/qt/resources/resources_ui.qrc
+++ b/lib/ui/asset/qt/resources/resources_ui.qrc
@@ -136,6 +136,7 @@
 	<file>sprite/windows_icon.png</file>
 	<file>stylesheet_ui.css</file>
 	<file>stylesheet_ui_linux.css</file>
+	<file>stylesheet_ui_mac.css</file>
 	<file>stylesheet_ui_windows.css</file>
 	<file>video/video_intro.gif</file>
 </qresource>
diff --git a/lib/ui/asset/qt/resources/stylesheet_ui_mac.css b/lib/ui/asset/qt/resources/stylesheet_ui_mac.css
new file mode 100644
index 000000000..e69de29bb
diff --git a/lib/ui/include/ui/qt/main_window.hpp b/lib/ui/include/ui/qt/main_window.hpp
index a22a720bb..745e19229 100644
--- a/lib/ui/include/ui/qt/main_window.hpp
+++ b/lib/ui/include/ui/qt/main_window.hpp
@@ -67,7 +67,7 @@ namespace VTX::UI::QT
 
 		void addFloatingWindow( QDialog * const p_window, const QSize & p_size, const bool p_visible );
 
-		void appendStylesheet( const char * p_stylesheetPath );
+		// void appendStylesheet( const char * p_stylesheetPath );
 
 	  protected:
 		void _setupUi( const QString & p_name ) override;
@@ -85,8 +85,8 @@ namespace VTX::UI::QT
 		void closeEvent( QCloseEvent * const p_event ) override;
 
 	  private:
-		QT::Widget::MainMenu::MainMenuBar *		 _mainMenuBar;
-		QT::Widget::Status::StatusBar* _statusBar;
+		QT::Widget::MainMenu::MainMenuBar * _mainMenuBar;
+		QT::Widget::Status::StatusBar *		_statusBar;
 
 		// Actions.
 		void _onDockWindowVisibilityChange( bool p_visible );
@@ -94,7 +94,7 @@ namespace VTX::UI::QT
 		void _updatePicker() const;
 
 		// Functions.
-		void _loadStyleSheet( const char * p_stylesheetPath );
+		void _loadTheme();
 
 		Core::WindowMode _getWindowModeFromWindowState( const Qt::WindowStates & p_state );
 		std::string		 _getWindowTitle() const;
diff --git a/lib/ui/include/ui/theme/theme.hpp b/lib/ui/include/ui/theme/theme.hpp
new file mode 100644
index 000000000..ad8d9a78b
--- /dev/null
+++ b/lib/ui/include/ui/theme/theme.hpp
@@ -0,0 +1,9 @@
+#include <string>
+
+namespace VTX::UI::Theme
+{
+	struct Theme
+	{
+		std::string color;
+	};
+} // namespace VTX::UI::Theme
diff --git a/lib/ui/src/ui/qt/main_window.cpp b/lib/ui/src/ui/qt/main_window.cpp
index 269b597f7..24e470c75 100644
--- a/lib/ui/src/ui/qt/main_window.cpp
+++ b/lib/ui/src/ui/qt/main_window.cpp
@@ -31,7 +31,7 @@ namespace VTX::UI::QT
 
 		setDockOptions( DockOption::VerticalTabs | DockOption::AllowNestedDocks | DockOption::AllowTabbedDocks );
 
-		_loadStyleSheet( ":/stylesheet_ui.css" );
+		_loadTheme();
 	}
 
 	void MainWindow::initWindowLayout() {}
@@ -49,15 +49,37 @@ namespace VTX::UI::QT
 		);
 	}
 
-	void MainWindow::_loadStyleSheet( const char * p_stylesheetPath )
+	void MainWindow::_loadTheme()
 	{
-		QFile stylesheetFile( p_stylesheetPath );
+		// TODO: move to style constants.
+		// Load main stylesheet.
+		QFile stylesheetFile( ":/stylesheet_ui.css" );
 		stylesheetFile.open( QFile::ReadOnly );
 
-		const QString stylesheet = stylesheetFile.readAll();
+		QString stylesheet = stylesheetFile.readAll();
+
+		// Load os-specific stylesheet.
+#if _WIN32
+		QFile stylesheetOSFile( ":/stylesheet_ui_windows.css" );
+#elif __linux__
+		QFile stylesheetOSFile( ":/stylesheet_ui_linux.css" );
+#elif __APPLE__
+		QFile stylesheetOSFile( ":/stylesheet_ui_mac.css" );
+#else
+		QFile stylesheetOSFile();
+		assert( true );
+#endif
+
+		stylesheetOSFile.open( QFile::ReadOnly );
+		stylesheet += '\n' + stylesheetOSFile.readAll();
+
+		// Load theme and apply to stylesheet.
+
+		// Set stylesheet to app.
 		setStyleSheet( stylesheet );
 	}
 
+	/*
 	void MainWindow::appendStylesheet( const char * p_stylesheetPath )
 	{
 		QFile stylesheetFile( p_stylesheetPath );
@@ -68,6 +90,7 @@ namespace VTX::UI::QT
 
 		setStyleSheet( newStylesheet );
 	}
+	*/
 
 	void MainWindow::_setupSlots() {}
 	void MainWindow::localize() { setWindowTitle( "VTX" ); }
diff --git a/lib/ui/src/ui/qt/tool/scene/widget/scene_tree_widget.cpp b/lib/ui/src/ui/qt/tool/scene/widget/scene_tree_widget.cpp
index 849ca9617..5ef3222d7 100644
--- a/lib/ui/src/ui/qt/tool/scene/widget/scene_tree_widget.cpp
+++ b/lib/ui/src/ui/qt/tool/scene/widget/scene_tree_widget.cpp
@@ -15,6 +15,12 @@ namespace VTX::UI::QT::Tool::Scene::Widget
 				static_cast<QTreeWidget *>( nullptr ), QStringList( QString( "item: %1" ).arg( i ) )
 			) );
 		insertTopLevelItems( 0, items );
+
+		/*
+		MAIN_REGISTRY().connectSignal<Component::Chemistry::Molecule, &ProxyMolecule::_removeFromRenderer>(
+			Core::ECS::SIGNAL::DESTROY, this
+		);
+		*/
 	}
 
 	void SceneTreeWidget::_setupSlots() {}
diff --git a/libraries/themes/Dark b/libraries/themes/Dark
new file mode 100644
index 000000000..1d127492e
--- /dev/null
+++ b/libraries/themes/Dark
@@ -0,0 +1,10 @@
+{
+    "DATA": {
+        "COLOR": #ffffff
+    },
+    "_VERSION": {
+        "MAJOR": 0,
+        "MINOR": 1,
+        "REVISION": 0
+    }
+}