LibGUI: Use LightGray as the base UI color.

This feels nicely reminiscent of the gap in time between Win3.11 and Win95,
one of my favorite eras in UI look-and-feel.

LibGUI/GTextBox.cpp

@@ -8,6 +8,7 @@
 GTextBox::GTextBox(GWidget* parent)
     : GWidget(parent)
 {
+    set_background_color(Color::White);
 }
 
 GTextBox::~GTextBox()