UI: Add ability to style preview background color

UI/data/themes/Acri.qss

@@ -880,3 +880,9 @@ FocusList::item {
 * [themeID="aboutHLayout"] {
 	background-color: rgb(8, 8, 11);
 }
+
+/* Preview background color */
+
+* [themeID="displayBackgroundColor"] {
+    qproperty-displayBackgroundColor: #28282A;
+}

UI/data/themes/Dark.qss

@@ -656,3 +656,9 @@ QLabel#errorLabel {
 * [themeID="aboutHLayout"] {
     background-color: rgb(31, 30, 31); /* veryDark */
 }
+
+/* Preview background color */
+
+* [themeID="displayBackgroundColor"] {
+    qproperty-displayBackgroundColor: rgb(76, 76, 76);
+}

UI/data/themes/Default.qss

@@ -135,3 +135,9 @@ QLabel#errorLabel {
 * [themeID="aboutHLayout"] {
 	background-color: rgb(169, 169, 169);
 }
+
+/* Preview background color */
+
+* [themeID="displayBackgroundColor"] {
+    qproperty-displayBackgroundColor: rgb(76, 76, 76);
+}

UI/data/themes/Rachni.qss

@@ -1234,3 +1234,9 @@ QToolTip {
 * [themeID="aboutHLayout"] {
 	background-color: rgb(35, 38, 41); /* Dark Gray */
 }
+
+/* Preview background color */
+
+* [themeID="displayBackgroundColor"] {
+    qproperty-displayBackgroundColor: rgb(35, 38, 41);
+}

UI/qt-display.cpp

@@ -6,6 +6,20 @@
 #include <QResizeEvent>
 #include <QShowEvent>
 
+static inline long long color_to_int(QColor color)
+{
+	auto shift = [&](unsigned val, int shift)
+	{
+		return ((val & 0xff) << shift);
+	};
+
+	return  shift(color.red(),    0) |
+		shift(color.green(),  8) |
+		shift(color.blue(),  16) |
+		shift(color.alpha(), 24);
+}
+
+
 OBSQTDisplay::OBSQTDisplay(QWidget *parent, Qt::WindowFlags flags)
 	: QWidget(parent, flags)
 {
@@ -39,6 +53,14 @@ OBSQTDisplay::OBSQTDisplay(QWidget *parent, Qt::WindowFlags flags)
 
 	connect(windowHandle(), &QWindow::visibleChanged, windowVisible);
 	connect(windowHandle(), &QWindow::screenChanged, sizeChanged);
+
+	this->setProperty("themeID", "displayBackgroundColor");
+}
+
+void OBSQTDisplay::SetDisplayBackgroundColor(const QColor &color)
+{
+	backgroundColor = (uint32_t)color_to_int(color);
+	obs_display_set_background_color(display, backgroundColor);
 }
 
 void OBSQTDisplay::CreateDisplay()
@@ -56,7 +78,7 @@ void OBSQTDisplay::CreateDisplay()
 
 	QTToGSWindow(winId(), info.window);
 
-	display = obs_display_create(&info);
+	display = obs_display_create(&info, backgroundColor);
 
 	emit DisplayCreated(this);
 }

UI/qt-display.hpp

@@ -5,6 +5,8 @@
 
 class OBSQTDisplay : public QWidget {
 	Q_OBJECT
+	Q_PROPERTY(QColor displayBackgroundColor WRITE SetDisplayBackgroundColor
+			NOTIFY SetDisplayBackgroundColor)
 
 	OBSDisplay display;
 
@@ -23,4 +25,9 @@ public:
 	virtual QPaintEngine *paintEngine() const override;
 
 	inline obs_display_t *GetDisplay() const {return display;}
+
+	uint32_t backgroundColor;
+
+private slots:
+	void SetDisplayBackgroundColor(const QColor &color);
 };