GH-1790 do not apply system theme on launch if it is selected

This prevents some ugly colors to show up on macOS in most cases.
It still looks ugly right after you switch to the it though.

application/MultiMC.cpp

@@ -355,7 +355,7 @@ MultiMC::MultiMC(int &argc, char **argv) : QApplication(argc, argv)
 	connect(this, SIGNAL(aboutToQuit()), SLOT(onExit()));
 
 	setIconTheme(settings()->get("IconTheme").toString());
-	setApplicationTheme(settings()->get("ApplicationTheme").toString());
+	setApplicationTheme(settings()->get("ApplicationTheme").toString(), true);
 
 	initAnalytics();
 
@@ -868,27 +868,14 @@ void MultiMC::initThemes()
 	insertTheme(new CustomTheme(darkTheme, "custom"));
 }
 
-void MultiMC::setApplicationTheme(const QString& name)
+void MultiMC::setApplicationTheme(const QString& name, bool initial)
 {
 	auto systemPalette = qApp->palette();
 	auto themeIter = m_themes.find(name);
 	if(themeIter != m_themes.end())
 	{
 		auto & theme = (*themeIter).second;
-		setStyle(QStyleFactory::create(theme->qtTheme()));
-		if(theme->hasColorScheme())
-		{
-			setPalette(theme->colorScheme());
-		}
-		if(theme->hasStyleSheet())
-		{
-			setStyleSheet(theme->appStyleSheet());
-		}
-		else
-		{
-			setStyleSheet(QString());
-		}
-		QDir::setSearchPaths("theme", theme->searchPaths());
+		theme->apply(initial);
 	}
 	else
 	{

application/MultiMC.h

@@ -87,7 +87,7 @@ public:
 
 	std::vector<ITheme *> getValidApplicationThemes();
 
-	void setApplicationTheme(const QString& name);
+	void setApplicationTheme(const QString& name, bool initial);
 
 	// DownloadUpdateTask
 	std::shared_ptr<UpdateChecker> updateChecker()

application/pages/global/MultiMCPage.cpp

@@ -330,7 +330,7 @@ void MultiMCPage::applySettings()
 	if(originalAppTheme != newAppTheme)
 	{
 		s->set("ApplicationTheme", newAppTheme);
-		MMC->setApplicationTheme(newAppTheme);
+		MMC->setApplicationTheme(newAppTheme, false);
 	}
 
 	// Console settings

application/themes/ITheme.cpp

@@ -1,5 +1,26 @@
 #include "ITheme.h"
 #include "rainbow.h"
+#include <QStyleFactory>
+#include <QDir>
+#include "MultiMC.h"
+
+void ITheme::apply(bool)
+{
+	QApplication::setStyle(QStyleFactory::create(qtTheme()));
+	if(hasColorScheme())
+	{
+		QApplication::setPalette(colorScheme());
+	}
+	if(hasStyleSheet())
+	{
+		MMC->setStyleSheet(appStyleSheet());
+	}
+	else
+	{
+		MMC->setStyleSheet(QString());
+	}
+	QDir::setSearchPaths("theme", searchPaths());
+}
 
 QPalette ITheme::fadeInactive(QPalette in, qreal bias, QColor color)
 {

application/themes/ITheme.h

@@ -8,6 +8,7 @@ class ITheme
 {
 public:
 	virtual ~ITheme() {}
+	virtual void apply(bool initial);
 	virtual QString id() = 0;
 	virtual QString name() = 0;
 	virtual bool hasStyleSheet() = 0;

application/themes/SystemTheme.cpp

@@ -24,6 +24,16 @@ SystemTheme::SystemTheme()
 	qWarning() << "System theme not found, defaulted to Fusion";
 }
 
+void SystemTheme::apply(bool initial)
+{
+	// if we are applying the system theme as the first theme, just don't touch anything. it's for the better...
+	if(initial)
+	{
+		return;
+	}
+	ITheme::apply(initial);
+}
+
 QString SystemTheme::id()
 {
 	return "system";
@@ -66,10 +76,5 @@ bool SystemTheme::hasStyleSheet()
 
 bool SystemTheme::hasColorScheme()
 {
-	// FIXME: horrible hack to work around Qt's sketchy theming APIs
-#if defined(Q_OS_LINUX)
 	return true;
-#else
-	return false;
-#endif
 }

application/themes/SystemTheme.h

@@ -7,6 +7,7 @@ class SystemTheme: public ITheme
 public:
 	SystemTheme();
 	virtual ~SystemTheme() {}
+	void apply(bool initial) override;
 
 	QString id() override;
 	QString name() override;