Fix GUI dark theme link text color visibility

This sets the application's palette color for links to the shade of blue
from gridcoin-community#847. It improves the visibility of links when using the dark mode.

Author: cyrossignol

src/qt/bitcoingui.cpp

@@ -143,6 +143,8 @@ BitcoinGUI::BitcoinGUI(QWidget *parent):
     QPalette pal(qApp->palette());
     pal.setColor(QPalette::Base, QColor(0, 0, 0, 0));
     pal.setColor(QPalette::Mid, pal.color(QPalette::Base));
+    pal.setColor(QPalette::Link, QColor(73, 144, 226));
+    pal.setColor(QPalette::LinkVisited, QColor(73, 144, 226));
     qApp->setPalette(pal);
 
     setWindowTitle(tr("Gridcoin") + " " + tr("Wallet"));