diff --git a/include/FramelessHelper/Widgets/framelesswidgetshelper.h b/include/FramelessHelper/Widgets/framelesswidgetshelper.h
index c155a81..57d3868 100644
--- a/include/FramelessHelper/Widgets/framelesswidgetshelper.h
+++ b/include/FramelessHelper/Widgets/framelesswidgetshelper.h
@@ -26,7 +26,7 @@
 
 #include "framelesshelperwidgets_global.h"
 #include <QtCore/qobject.h>
-#include <QtGui/qwindowdefs.h>
+#include <QtGui/qwindow.h>
 
 QT_BEGIN_NAMESPACE
 class QLabel;
diff --git a/src/core/CMakeLists.txt b/src/core/CMakeLists.txt
index 4cb03a7..2190126 100644
--- a/src/core/CMakeLists.txt
+++ b/src/core/CMakeLists.txt
@@ -22,6 +22,11 @@
   SOFTWARE.
 ]]
 
+if(UNIX AND NOT APPLE)
+    find_package(GTK3 REQUIRED)
+    find_package(X11 REQUIRED)
+endif()
+
 set(SUB_PROJ_NAME FramelessHelperCore)
 
 set(INCLUDE_PREFIX ../../include/FramelessHelper/Core)
@@ -102,6 +107,19 @@ else()
     )
 endif()
 
+if(UNIX AND NOT APPLE)
+    target_compile_definitions(${SUB_PROJ_NAME} PRIVATE
+        GDK_VERSION_MIN_REQUIRED=GDK_VERSION_3_6
+    )
+    target_link_libraries(${SUB_PROJ_NAME} PRIVATE
+        PkgConfig::GTK3
+        X11::X11
+    )
+    target_include_directories(${SUB_PROJ_NAME} PRIVATE
+        "/usr/include/harfbuzz" # GCC complains about can't find "hb.h". Why do we need this?
+    )
+endif()
+
 target_link_libraries(${SUB_PROJ_NAME} PRIVATE
     Qt${QT_VERSION_MAJOR}::CorePrivate
     Qt${QT_VERSION_MAJOR}::GuiPrivate
diff --git a/src/core/utils_linux.cpp b/src/core/utils_linux.cpp
index bfd034e..21726e1 100644
--- a/src/core/utils_linux.cpp
+++ b/src/core/utils_linux.cpp
@@ -33,7 +33,7 @@
 #if ((QT_VERSION >= QT_VERSION_CHECK(5, 9, 1)) && (QT_VERSION < QT_VERSION_CHECK(6, 0, 0)))
 #  include <QtPlatformHeaders/qxcbscreenfunctions.h>
 #endif
-//#include <gtk/gtk.h>
+#include <gtk/gtk.h>
 #include <X11/Xlib.h>
 
 FRAMELESSHELPER_BEGIN_NAMESPACE
@@ -52,11 +52,16 @@ static constexpr const auto _NET_WM_MOVERESIZE_SIZE_LEFT        = 7;
 static constexpr const auto _NET_WM_MOVERESIZE_MOVE             = 8;
 #endif
 
+static constexpr const char GTK_THEME_NAME_ENV_VAR[] = "GTK_THEME";
+static constexpr const char GTK_THEME_NAME_PROP[] = "gtk-theme-name";
+static constexpr const char GTK_THEME_PREFER_DARK_PROP[] = "gtk-application-prefer-dark-theme";
+
 FRAMELESSHELPER_BYTEARRAY_CONSTANT(display)
 FRAMELESSHELPER_BYTEARRAY_CONSTANT(x11screen)
 FRAMELESSHELPER_BYTEARRAY_CONSTANT(rootwindow)
 
-#if 0
+FRAMELESSHELPER_STRING_CONSTANT2(GTK_THEME_DARK_REGEX, "[:-]dark")
+
 template<typename T>
 [[nodiscard]] static inline T gtkSetting(const gchar *propertyName)
 {
@@ -80,12 +85,11 @@ template<typename T>
     if (!propertyName) {
         return {};
     }
-    const auto value = gtkSetting<gchararray>(propertyName);
-    const QString result = QString::fromUtf8(value);
-    g_free(value);
+    const auto propertyValue = gtkSetting<gchararray>(propertyName);
+    const QString result = QString::fromUtf8(propertyValue);
+    g_free(propertyValue);
     return result;
 }
-#endif
 
 #if (QT_VERSION < QT_VERSION_CHECK(5, 15, 0))
 [[nodiscard]] static inline Qt::WindowFrameSection qtEdgesToQtWindowFrameSection(const Qt::Edges edges)
@@ -123,7 +127,6 @@ template<typename T>
 
 [[nodiscard]] bool shouldAppsUseDarkMode_linux()
 {
-#if 0
     /*
         https://docs.gtk.org/gtk3/running.html
 
@@ -137,10 +140,10 @@ template<typename T>
         it's mainly used for easy debugging, so it should be possible to use it
         to override any other settings.
     */
-    QString themeName = qEnvironmentVariable("GTK_THEME");
-    const QRegularExpression darkRegex(QStringLiteral("[:-]dark"), QRegularExpression::CaseInsensitiveOption);
-    if (!themeName.isEmpty()) {
-        return darkRegex.match(themeName).hasMatch();
+    static const QRegularExpression darkRegex(kGTK_THEME_DARK_REGEX, QRegularExpression::CaseInsensitiveOption);
+    const QString envThemeName = qEnvironmentVariable(GTK_THEME_NAME_ENV_VAR);
+    if (!envThemeName.isEmpty()) {
+        return darkRegex.match(envThemeName).hasMatch();
     }
 
     /*
@@ -150,7 +153,7 @@ template<typename T>
         gtk-theme-name provides both light and dark variants. We can save a
         regex check by testing this property first.
     */
-    const auto preferDark = gtkSetting<bool>("gtk-application-prefer-dark-theme");
+    const auto preferDark = gtkSetting<bool>(GTK_THEME_PREFER_DARK_PROP);
     if (preferDark) {
         return true;
     }
@@ -158,15 +161,12 @@ template<typename T>
     /*
         https://docs.gtk.org/gtk3/property.Settings.gtk-theme-name.html
     */
-    themeName = gtkSetting("gtk-theme-name");
-    if (!themeName.isEmpty()) {
-        return darkRegex.match(themeName).hasMatch();
+    const QString curThemeName = gtkSetting(GTK_THEME_NAME_PROP);
+    if (!curThemeName.isEmpty()) {
+        return darkRegex.match(curThemeName).hasMatch();
     }
 
     return false;
-#else
-    return false;
-#endif
 }
 
 #if (QT_VERSION < QT_VERSION_CHECK(5, 15, 0))
diff --git a/src/quick/quickstandardclosebutton.cpp b/src/quick/quickstandardclosebutton.cpp
index 56dcafe..e23f70b 100644
--- a/src/quick/quickstandardclosebutton.cpp
+++ b/src/quick/quickstandardclosebutton.cpp
@@ -29,7 +29,6 @@
 #include <QtQuick/private/qquickimage_p.h>
 #include <QtQuick/private/qquickrectangle_p.h>
 #include <QtQuick/private/qquickanchors_p.h>
-#include <QtQuickTemplates2/private/qquicktooltip_p.h>
 
 static inline void initResource()
 {
diff --git a/src/quick/quickstandardclosebutton_p.h b/src/quick/quickstandardclosebutton_p.h
index 875b509..b37b40e 100644
--- a/src/quick/quickstandardclosebutton_p.h
+++ b/src/quick/quickstandardclosebutton_p.h
@@ -24,13 +24,13 @@
 
 #pragma once
 
-#include <QtQuickTemplates2/private/qquickbutton_p.h>
 #include "framelesshelperquick_global.h"
+#include <QtQuickTemplates2/private/qquicktooltip_p.h>
+#include <QtQuickTemplates2/private/qquickbutton_p.h>
 
 QT_BEGIN_NAMESPACE
 class QQuickImage;
 class QQuickRectangle;
-class QQuickToolTipAttached;
 QT_END_NAMESPACE
 
 FRAMELESSHELPER_BEGIN_NAMESPACE
diff --git a/src/quick/quickstandardmaximizebutton.cpp b/src/quick/quickstandardmaximizebutton.cpp
index 7446901..c0ffce4 100644
--- a/src/quick/quickstandardmaximizebutton.cpp
+++ b/src/quick/quickstandardmaximizebutton.cpp
@@ -29,7 +29,6 @@
 #include <QtQuick/private/qquickimage_p.h>
 #include <QtQuick/private/qquickrectangle_p.h>
 #include <QtQuick/private/qquickanchors_p.h>
-#include <QtQuickTemplates2/private/qquicktooltip_p.h>
 
 static inline void initResource()
 {
diff --git a/src/quick/quickstandardmaximizebutton_p.h b/src/quick/quickstandardmaximizebutton_p.h
index 85865a3..d256dfc 100644
--- a/src/quick/quickstandardmaximizebutton_p.h
+++ b/src/quick/quickstandardmaximizebutton_p.h
@@ -24,13 +24,13 @@
 
 #pragma once
 
-#include <QtQuickTemplates2/private/qquickbutton_p.h>
 #include "framelesshelperquick_global.h"
+#include <QtQuickTemplates2/private/qquickbutton_p.h>
+#include <QtQuickTemplates2/private/qquicktooltip_p.h>
 
 QT_BEGIN_NAMESPACE
 class QQuickImage;
 class QQuickRectangle;
-class QQuickToolTipAttached;
 QT_END_NAMESPACE
 
 FRAMELESSHELPER_BEGIN_NAMESPACE
diff --git a/src/quick/quickstandardminimizebutton.cpp b/src/quick/quickstandardminimizebutton.cpp
index 98c92a1..9b5885f 100644
--- a/src/quick/quickstandardminimizebutton.cpp
+++ b/src/quick/quickstandardminimizebutton.cpp
@@ -29,7 +29,6 @@
 #include <QtQuick/private/qquickimage_p.h>
 #include <QtQuick/private/qquickrectangle_p.h>
 #include <QtQuick/private/qquickanchors_p.h>
-#include <QtQuickTemplates2/private/qquicktooltip_p.h>
 
 static inline void initResource()
 {
diff --git a/src/quick/quickstandardminimizebutton_p.h b/src/quick/quickstandardminimizebutton_p.h
index 6a7c609..17dee5a 100644
--- a/src/quick/quickstandardminimizebutton_p.h
+++ b/src/quick/quickstandardminimizebutton_p.h
@@ -24,13 +24,13 @@
 
 #pragma once
 
-#include <QtQuickTemplates2/private/qquickbutton_p.h>
 #include "framelesshelperquick_global.h"
+#include <QtQuickTemplates2/private/qquickbutton_p.h>
+#include <QtQuickTemplates2/private/qquicktooltip_p.h>
 
 QT_BEGIN_NAMESPACE
 class QQuickImage;
 class QQuickRectangle;
-class QQuickToolTipAttached;
 QT_END_NAMESPACE
 
 FRAMELESSHELPER_BEGIN_NAMESPACE
diff --git a/src/widgets/framelesswidgetshelper.cpp b/src/widgets/framelesswidgetshelper.cpp
index 76c78bd..d8798c2 100644
--- a/src/widgets/framelesswidgetshelper.cpp
+++ b/src/widgets/framelesswidgetshelper.cpp
@@ -27,7 +27,6 @@
 #include <QtCore/qdebug.h>
 #include <QtGui/qpainter.h>
 #include <QtGui/qevent.h>
-#include <QtGui/qwindow.h>
 #include <QtWidgets/qboxlayout.h>
 #include <QtWidgets/qlabel.h>
 #include <framelesswindowsmanager.h>