Add custom application color picker in global settings

Add a KColorButton next to the 'Utiliser les couleurs du système'
checkbox in the Apparence settings tab:

- When the checkbox is active (default), system colors are used
  and the color button is disabled
- When the checkbox is inactive, the color button becomes active
  and lets the user pick any color for the entire application
- useCustomPalette() builds a full QPalette from the chosen color
  with proper light/dark text contrast, button shading, and icon
  theme switching
- The chosen color is persisted in QSettings as
  'customapplicationcolor' and restored on next startup

Files changed:
- sources/ui/configpage/generalconfigurationpage.ui: HBoxLayout
  with checkbox + KColorButton, customwidget declaration
- sources/ui/configpage/generalconfigurationpage.h: new slot
- sources/ui/configpage/generalconfigurationpage.cpp: load/save
  custom color, enable/disable logic, toggled slot
- sources/qetapp.h: useCustomPalette() declaration
- sources/qetapp.cpp: useCustomPalette() implementation,
  startup restore of custom color

sources/qetapp.cpp

@@ -1816,6 +1816,81 @@ void QETApp::useSystemPalette(bool use) {
 	QET::Palette::refreshStyleSheets();
 }
 
+/**
+	@brief QETApp::useCustomPalette
+	Apply a user-chosen color as the application-wide palette.
+	Builds a full QPalette from \a color, keeping the system palette
+	as a fallback for roles we don't touch.
+	@param color the user-chosen base color
+*/
+void QETApp::useCustomPalette(const QColor &color) {
+	if (!color.isValid())
+		return;
+
+	// Derive readable text colors from the chosen color.
+	const bool dark = color.lightness() < 128;
+	const QColor text = dark ? QColor(220, 220, 220) : QColor(30, 30, 30);
+	const QColor disabled_text = dark ? QColor(175, 175, 175) : QColor(128, 128, 128);
+
+	// Slightly lighter/darker for button and window shading.
+	QColor button = color;
+	button = QColor::fromHslF(color.hslHueF(),
+				  color.hslSaturationF(),
+				  dark ? qMin(color.lightnessF() + 0.08, 1.0)
+				       : qMax(color.lightnessF() - 0.08, 0.0));
+	QColor light = QColor::fromHslF(color.hslHueF(),
+					color.hslSaturationF(),
+					dark ? qMin(color.lightnessF() + 0.15, 1.0)
+					     : qMax(color.lightnessF() - 0.15, 0.0));
+	QColor mid = QColor::fromHslF(color.hslHueF(),
+				     color.hslSaturationF(),
+				     dark ? qMin(color.lightnessF() + 0.04, 1.0)
+				          : qMax(color.lightnessF() - 0.04, 0.0));
+	QColor dark_c = QColor::fromHslF(color.hslHueF(),
+					 color.hslSaturationF(),
+					 dark ? qMin(color.lightnessF() - 0.04, 1.0)
+					      : qMax(color.lightnessF() - 0.12, 0.0));
+	QColor shadow = QColor::fromHslF(color.hslHueF(),
+					 color.hslSaturationF(),
+					 dark ? qMin(color.lightnessF() - 0.10, 1.0)
+					      : qMax(color.lightnessF() - 0.20, 0.0));
+
+	QPalette p;
+	// Active and Inactive get the same colors; only Disabled differs.
+	for (auto group : {QPalette::Active, QPalette::Inactive}) {
+		p.setColor(group, QPalette::Window,          color);
+		p.setColor(group, QPalette::WindowText,      text);
+		p.setColor(group, QPalette::Base,            color);
+		p.setColor(group, QPalette::AlternateBase,   button);
+		p.setColor(group, QPalette::Text,            text);
+		p.setColor(group, QPalette::Button,          button);
+		p.setColor(group, QPalette::ButtonText,      text);
+		p.setColor(group, QPalette::BrightText,      dark ? QColor(255,90,90) : Qt::white);
+		p.setColor(group, QPalette::Highlight,       QColor(30, 96, 176));
+		p.setColor(group, QPalette::HighlightedText, Qt::white);
+		p.setColor(group, QPalette::ToolTipBase,     button);
+		p.setColor(group, QPalette::ToolTipText,     text);
+		p.setColor(group, QPalette::Light,           light);
+		p.setColor(group, QPalette::Midlight,        mid);
+		p.setColor(group, QPalette::Mid,             mid);
+		p.setColor(group, QPalette::Dark,            dark_c);
+		p.setColor(group, QPalette::Shadow,          shadow);
+#if QT_VERSION >= QT_VERSION_CHECK(6, 6, 0)
+		p.setColor(group, QPalette::Accent,          QColor(30, 96, 176));
+#endif
+	}
+	p.setColor(QPalette::Disabled, QPalette::WindowText, disabled_text);
+	p.setColor(QPalette::Disabled, QPalette::Text,       disabled_text);
+	p.setColor(QPalette::Disabled, QPalette::ButtonText, disabled_text);
+
+	qApp->setPalette(p);
+	qApp->setStyleSheet(QString());
+
+	// Switch icon theme to match light/dark.
+	applyIconTheme(p);
+	QET::Palette::refreshStyleSheets();
+}
+
 /**
 	@brief QETApp::quitQET
 	Request the closing of all windows;
@@ -2404,7 +2479,13 @@ void QETApp::initStyle()
 
 	//Apply or not the system style
 	QSettings settings;
-	useSystemPalette(settings.value("usesystemcolors", true).toBool());
+	if (settings.value("usesystemcolors", true).toBool()) {
+		useSystemPalette(true);
+	} else if (settings.contains("customapplicationcolor")) {
+		useCustomPalette(QColor(settings.value("customapplicationcolor").toString()));
+	} else {
+		useSystemPalette(false);
+	}
 
 #if defined(Q_OS_MACOS) && QT_VERSION >= QT_VERSION_CHECK(6, 5, 0)
 	// Setting an application palette stops Qt from following the OS

sources/qetapp.h

@@ -263,6 +263,7 @@ class QETApp : public QObject
 		void setMainWindowVisible(QMainWindow *, bool);
 		void invertMainWindowVisibility(QWidget *);
 		void useSystemPalette(bool);
+		void useCustomPalette(const QColor &color);
 		void quitQET();
 		void checkRemainingWindows();
 		void openFiles(const QETArguments &);

sources/qetpalette.cpp

@@ -20,7 +20,9 @@
 #include <QApplication>
 #include <QColor>
 #include <QImage>
+#include <QMdiArea>
 #include <QStyle>
+#include <QTabBar>
 #include <QWidget>
 #include <cmath>
 
@@ -307,4 +309,10 @@ void QET::Palette::refreshStyleSheets()
 	for (QWidget *widget : widgets)
 		if (!widget->styleSheet().isEmpty())
 			widget->setStyleSheet(widget->styleSheet());
+
+	// Force an immediate repaint on tab bars and MDI areas so their text
+	// updates together with the rest of the UI, not one event loop later.
+	for (QWidget *widget : widgets)
+		if (qobject_cast<QTabBar *>(widget) || qobject_cast<QMdiArea *>(widget))
+			widget->update();
 }

sources/ui/configpage/generalconfigurationpage.cpp

@@ -23,6 +23,7 @@
 #include "../../utils/qetsettings.h"
 #include "../../utils/qetutils.h"
 #include "../../qetmessagebox.h"
+#include "../nokde/kcolorbutton.h"
 #include <QFileDialog>
 #include <QFontDialog>
 #include <QSettings>
@@ -74,6 +75,12 @@ GeneralConfigurationPage::GeneralConfigurationPage(QWidget *parent) :
 	ui->DiagramEditor_Grid_PointSize_min_sb->setValue(settings.value("diagrameditor/grid_pointsize_min", 1).toInt());
 	ui->DiagramEditor_Grid_PointSize_max_sb->setValue(settings.value("diagrameditor/grid_pointsize_max", 1).toInt());
 	ui->m_use_system_color_cb->setChecked(settings.value("usesystemcolors", "true").toBool());
+	bool sysColors = ui->m_use_system_color_cb->isChecked();
+	ui->m_custom_app_color_kpb->setEnabled(!sysColors);
+	if (settings.contains("customapplicationcolor"))
+		ui->m_custom_app_color_kpb->setColor(QColor(settings.value("customapplicationcolor").toString()));
+	else
+		ui->m_custom_app_color_kpb->setColor(QApplication::palette().color(QPalette::Window));
 	bool tabbed = settings.value("diagrameditor/viewmode", "tabbed") == "tabbed";
 	if(tabbed)
 		ui->m_use_tab_mode_rb->setChecked(true);
@@ -206,7 +213,17 @@ void GeneralConfigurationPage::applyConf()
 	bool must_use_system_colors  = ui->m_use_system_color_cb->isChecked();
 	settings.setValue("usesystemcolors", must_use_system_colors);
 	if (was_using_system_colors != must_use_system_colors) {
-		QETApp::instance()->useSystemPalette(must_use_system_colors);
+		if (must_use_system_colors) {
+			QETApp::instance()->useSystemPalette(true);
+		} else {
+			QColor custom_color = ui->m_custom_app_color_kpb->color();
+			settings.setValue("customapplicationcolor", custom_color.name());
+			QETApp::instance()->useCustomPalette(custom_color);
+		}
+	} else if (!must_use_system_colors) {
+		QColor custom_color = ui->m_custom_app_color_kpb->color();
+		settings.setValue("customapplicationcolor", custom_color.name());
+		QETApp::instance()->useCustomPalette(custom_color);
 	}
 	settings.setValue("border-columns_0",ui->m_border_0->isChecked());
 	settings.setValue("lang", ui->m_lang_cb->itemData(ui->m_lang_cb->currentIndex()).toString());
@@ -625,3 +642,14 @@ void GeneralConfigurationPage::on_m_hdpi_round_cb_clicked(bool checked)
 	ui->m_hdpi_round_policy_cb->setEnabled(checked);
 }
 
+/**
+	@brief GeneralConfigurationPage::on_m_use_system_color_cb_toggled
+	Enable/disable the custom color picker when the system color
+	checkbox is toggled.
+	@param checked
+*/
+void GeneralConfigurationPage::on_m_use_system_color_cb_toggled(bool checked)
+{
+	ui->m_custom_app_color_kpb->setEnabled(!checked);
+}
+

sources/ui/configpage/generalconfigurationpage.h

@@ -53,6 +53,7 @@ class GeneralConfigurationPage : public ConfigPage
 		void on_ElementEditor_Grid_PointSize_min_sb_valueChanged(int value);
 
 		void on_m_hdpi_round_cb_clicked(bool checked);
+		void on_m_use_system_color_cb_toggled(bool checked);
 
 	private:
 		void fillLang();

sources/ui/configpage/generalconfigurationpage.ui

@@ -23,15 +23,26 @@
       <attribute name="title">
        <string>Apparence</string>
       </attribute>
-      <layout class="QVBoxLayout" name="verticalLayout">
-       <item>
-        <widget class="QCheckBox" name="m_use_system_color_cb">
-         <property name="text">
-          <string>Utiliser les couleurs du système</string>
-         </property>
-        </widget>
-       </item>
-       <item>
+       <layout class="QVBoxLayout" name="verticalLayout">
+        <item>
+         <layout class="QHBoxLayout" name="m_system_color_layout">
+          <item>
+           <widget class="QCheckBox" name="m_use_system_color_cb">
+            <property name="text">
+             <string>Utiliser les couleurs du système</string>
+            </property>
+           </widget>
+          </item>
+          <item>
+           <widget class="KColorButton" name="m_custom_app_color_kpb">
+            <property name="toolTip">
+             <string>Couleur de l'application</string>
+            </property>
+           </widget>
+          </item>
+         </layout>
+        </item>
+        <item>
         <widget class="Line" name="line">
          <property name="orientation">
           <enum>Qt::Horizontal</enum>
@@ -1161,4 +1172,11 @@ Vous pouvez spécifier ici la valeur par défaut de ce champ pour les éléments
  </tabstops>
  <resources/>
  <connections/>
+ <customwidgets>
+  <customwidget>
+   <class>KColorButton</class>
+   <extends>QPushButton</extends>
+   <header>nokde/kcolorbutton.h</header>
+  </customwidget>
+ </customwidgets>
 </ui>