Support accent colors if a theme specifies that it can use them + add two example themes for light and dark. #216
Description
The example themes could probably just be called "Variable Accent Light" and "Variable Accent Dark", with their filenames being "VAccentLight" and "VAccentDark". These two themes would have accent colors show up when a button is pressed down, in the menuitem image area, for linklabel link colors, and groupbox header text, just to name a few. These will probably also have Metro-ish buttons if possible, just because and also because it makes sense if we're using accent colors on button press. Fancy animations will not be included, this is for Windows Forms.
Themes would have a new argument (?) for each element that would be like UseAccentColor = True if the theme wants that element to use an accent color, with a default one settable in the theme's details section so that the user can choose whether to use the default accent color a theme provides, or override it with their own choice.