Full list of supported X resources
I've only used the colors and scheme, so I can't say how well the others work.
* background - The default background color for widgets (color).
* dndTextOps - The default setting for drag and drop text operations (boolean).
* foreground - The default foreground (label) color for widgets (color).
* scheme - The default scheme to use (string).
* selectBackground - The default selection color for menus, etc. (color).
* Text.background - The default background color for text fields (color).
* tooltips - The default setting for tooltips (boolean).
* visibleFocus - The default setting for visible keyboard focus on non-text widgets (boolean).
Resources associated with the first window's Fl_Window::xclass() string are queried first, or if no class has been specified then the class "fltk" is used (e.g. fltk.background). If no match is found, a global search is done (e.g. *background).