Use 'system' theme as default if QT_QPA_PLATFORMTHEME is set

On first launch, before the user has configured any settings,
check the value of the QT_QPA_PLATFORMTHEME environment var.
If it is set, use the system theme as the default instead of the
light theme.  This fixes #72.

src/UserSettingsPage.cpp

@@ -22,9 +22,11 @@
 #include <QLabel>
 #include <QLineEdit>
 #include <QMessageBox>
+#include <QProcessEnvironment>
 #include <QPushButton>
 #include <QScrollArea>
 #include <QSettings>
+#include <QString>
 #include <QTextStream>
 
 #include "Config.h"
@@ -49,7 +51,7 @@ UserSettings::load()
         isGroupViewEnabled_           = settings.value("user/group_view", true).toBool();
         isTypingNotificationsEnabled_ = settings.value("user/typing_notifications", true).toBool();
         isReadReceiptsEnabled_        = settings.value("user/read_receipts", true).toBool();
-        theme_                        = settings.value("user/theme", "light").toString();
+        theme_                        = settings.value("user/theme", defaultTheme_).toString();
         font_                         = settings.value("user/font_family", "default").toString();
         emojiFont_    = settings.value("user/emoji_font_family", "default").toString();
         baseFontSize_ = settings.value("user/font_size", QFont().pointSizeF()).toDouble();

src/UserSettingsPage.h

@@ -85,7 +85,7 @@ public:
                 save();
         }
 
-        QString theme() const { return !theme_.isEmpty() ? theme_ : "light"; }
+        QString theme() const { return !theme_.isEmpty() ? theme_ : defaultTheme_; }
         bool isTrayEnabled() const { return isTrayEnabled_; }
         bool isStartInTrayEnabled() const { return isStartInTrayEnabled_; }
         bool isGroupViewEnabled() const { return isGroupViewEnabled_; }
@@ -100,6 +100,11 @@ signals:
         void groupViewStateChanged(bool state);
 
 private:
+        // Default to system theme if QT_QPA_PLATFORMTHEME var is set.
+        QString defaultTheme_ =
+          QProcessEnvironment::systemEnvironment().value("QT_QPA_PLATFORMTHEME", "").isEmpty()
+            ? "light"
+            : "system";
         QString theme_;
         bool isTrayEnabled_;
         bool isStartInTrayEnabled_;

src/Utils.cpp

@@ -4,6 +4,7 @@
 #include <QComboBox>
 #include <QDesktopWidget>
 #include <QGuiApplication>
+#include <QProcessEnvironment>
 #include <QScreen>
 #include <QSettings>
 #include <QTextDocument>
@@ -387,14 +388,20 @@ QString
 utils::linkColor()
 {
         QSettings settings;
-        const auto theme = settings.value("user/theme", "light").toString();
-
-        if (theme == "light")
+        // Default to system theme if QT_QPA_PLATFORMTHEME var is set.
+        QString defaultTheme =
+          QProcessEnvironment::systemEnvironment().value("QT_QPA_PLATFORMTHEME", "").isEmpty()
+            ? "light"
+            : "system";
+        const auto theme = settings.value("user/theme", defaultTheme).toString();
+
+        if (theme == "light") {
                 return "#0077b5";
-        else if (theme == "dark")
+        } else if (theme == "dark") {
                 return "#38A3D8";
-
-        return QPalette().color(QPalette::Link).name();
+        } else {
+                return QPalette().color(QPalette::Link).name();
+        }
 }
 
 uint32_t