I think the challenge would be in coming up with an approach which won't make the terminal (non-gui) version drift from the GUI version too much. Changing the button styles etc in the GUI menu and perhaps updating toolbar icons etc is probably not too hard, but you cannot do much with things like 'buttons' in widgets etc (such as those used with customize) without having to have completely different code for rendering in GUI and rendering in terminal. This would potentially blow out maintenance and create two code bases to manage and lets face it, one is more than enough.

I think part of the reason the GUI menus and toolbar might look dated to many is that nearly all experienced and long-term users I know turn off the menus and toolbar, so never see them. It has been years since I've used either. For me, the only 'buttons' I see are the widget style buttons and these are not really buttons - they are really text 'fake' buttons. I would also be a little concerned about impact any attempt to change these might have on performance (we already have quite a long thread about rendering performance).

Just my 2 cents....