Window titlebars need to blend better with the window when there's no toolbar

[Christopher Kyle Horton]

Window titlebars should have their bottoms transition smoothly into the rest of the window. This is the case with windows with toolbars, but not for others like Calculator, Update Manager, etc. See attached screenshot for an example of how this visual disparity occurs.

The desired solution is likely to make the bottom portion of the titlebars' gradient darker, and alter the code for sidebars and toolbars appropriately to fit in with this change.