Minor theme fixes

* Support mouse hover color change for QPushButtons.
* Fix #5040 - don't enforce standard palette when in classic theme mode

src/gui/styles/base/basestyle.qss

@@ -3,6 +3,11 @@ QPushButton:default {
     color: palette(highlighted-text);
 }
 
+/* Note: default button hover is defined in the respective theme style */
+QPushButton:!default:hover {
+    background: palette(mid);
+}
+
 QSpinBox {
     min-width: 90px;
 }

src/gui/styles/dark/darkstyle.qss

@@ -8,6 +8,16 @@ EntryPreviewWidget QLineEdit:disabled, EntryPreviewWidget QTextEdit:disabled {
     background-color: #424242;
 }
 
+QPushButton:!default:hover {
+    /* Using slightly darker shade from palette(button) */
+    background: #252528;
+}
+
+QPushButton:default:hover {
+    /* Using slightly lighter shade from palette(highlight) */
+    background: #2E582E;
+}
+
 QToolTip {
     color: #BFBFBF;
     background-color: #2D532D;

src/gui/styles/light/lightstyle.qss

@@ -8,11 +8,16 @@ EntryPreviewWidget QLineEdit:disabled, EntryPreviewWidget QTextEdit:disabled {
     background-color: #EDEDED;
 }
 
-QGroupBox::title {
-    color: #4B7B19;
+QPushButton:default:hover {
+    /* Using slightly lighter shade from palette(highlight) */
+    background: #568821;
 }
 
 QToolTip {
     color: #F9F9F9;
     background-color: #4D7F1A;
 }
+
+QGroupBox::title {
+    color: #4B7B19;
+}