Use CSS style sheet.

Avoid confusing text behind default transparent popup.

PopupCertifyUserId.cpp

@@ -44,6 +44,8 @@ void PopupCertifyUserId::Create(vector<WString>& privateKeys,
 {
     m_fprKeyToSign = fprKeyToSign;
     m_cwMain = static_cast<WContainerWidget*> (implementation());
+    // White in css file, like default background color.
+    m_cwMain->setStyleClass("popup");
 
     WVBoxLayout * vblMain = new WVBoxLayout();
     m_cwMain->setLayout(unique_ptr<WVBoxLayout> (vblMain));