Fix styling for disabled buttons (#3205)

Author: UdjinM6

src/qt/res/css/light-hires.css

@@ -136,6 +136,10 @@ QPushButton:pressed {
 border:1px solid #333;
 }
 
+QPushButton:disabled {
+background-color: #666;
+}
+
 QComboBox { /* Dropdown Menus */
 border:1px solid #82C3E6;
 padding: 0px 5px 0px 5px;

src/qt/res/css/light.css

@@ -136,6 +136,10 @@ QPushButton:pressed {
 border:1px solid #333;
 }
 
+QPushButton:disabled {
+background-color: #666;
+}
+
 QComboBox { /* Dropdown Menus */
 border:1px solid #82C3E6;
 padding: 0px 5px 0px 5px;