Use the theme background color for panel line edits

Previously, the image used as background forced it to be white, which
made the text impossible to read when the palette foreground color
happened to be white as well.

Task-number: 258167

src/plugins/coreplugin/manhattanstyle.cpp

@@ -474,6 +474,12 @@ void ManhattanStyle::drawPrimitive(PrimitiveElement element, const QStyleOption
     case PE_PanelLineEdit:
         {
             painter->save();
+
+            // Fill the line edit background
+            QRect filledRect = option->rect.adjusted(1, 1, -1, -1);
+            painter->setBrushOrigin(filledRect.topLeft());
+            painter->fillRect(filledRect, option->palette.base());
+
             if (option->state & State_Enabled)
                 drawCornerImage(d->lineeditImage, painter, option->rect, 2, 2, 2, 2);
             else