css styles for mainHead and mainContent in widget.cpp

fixes problems with ugly UI on non-standard system theme.
before: https://i.imgur.com/XJ0mbpi.png
after: https://imgur.com/a/UbgNz

res.qrc

@@ -136,5 +136,7 @@
         <file>ui/chatroomWidgets/genericChatroomWidget.css</file>
         <file>ui/fileTransferInstance/sliverRTEdge.png</file>
         <file>ui/window/statusPanel.css</file>
+        <file>ui/settings/mainContent.css</file>
+        <file>ui/settings/mainHead.css</file>
     </qresource>
 </RCC>

ui/settings/mainContent.css

@@ -0,0 +1,36 @@
+QCheckBox
+{
+    color: black;
+}
+
+QLabel
+{
+    color: black;
+}
+
+QGroupBox::title
+{
+    color: black;
+    background-color: white;
+}
+
+QWidget
+{
+    color: black;
+    background-color: white;
+}
+
+QComboBox
+{
+    background-color: white;
+}
+
+QComboBox:on QComboBox:off QComboBox:drop-down
+{
+    background: rgba(18, 18, 18, 204);
+}
+
+QComboBox:active
+{
+    background: white;
+}

ui/settings/mainHead.css

@@ -0,0 +1,5 @@
+QWidget
+{
+    color: black;
+    background: white;
+}

widget/widget.cpp

@@ -74,6 +74,8 @@ Widget::Widget(QWidget *parent)
     ui->mainHead->setLayout(new QVBoxLayout());
     ui->mainHead->layout()->setMargin(0);
     ui->mainHead->layout()->setSpacing(0);
+    ui->mainHead->setStyleSheet(Style::getStylesheet(":ui/settings/mainHead.css"));
+    ui->mainContent->setStyleSheet(Style::getStylesheet(":ui/settings/mainContent.css"));
 
     contactListWidget = new FriendListWidget();
     ui->friendList->setWidget(contactListWidget);