Updates to Light Theme
This patch attempts to make some requested updates to the new Light theme from 8509.
*The Toolbox Wilber is now visible and colored correctly. This is done by changing the ".flat" styling to "button .flat" and adding the color to "GimpDock frame:first-child:not(label)"
*The large black border was removed from "Create a New Image"'s ScrolledWindow by defining a border radius and setting the border itself to 0px.
*A "duplicate text" effect appeared on buttons and labels when not selected. This was fixed by setting the text-shadow attribute to 0.
*The margins for the toolbox tabs were defined, to prevent them from moving/shifting when selected or unselected
*The "Add Tabs" menu in the toolbox and dockable dialogue column still had the old background. "menu" was added to the style list to provide the right background color
*An unnecessary black border around "Check Updates" box in the About Dialog was removed