Add per-display style, better displays

[bgribble]

I wanted a nice, flat 2d-look display... got that, but it's not all that satisfying. The black-white-red color scheme, no variation between items, no gradients at all... a bit dull (and confusing, frankly, since everything is so flat).

Here's a short list of improvements:

• "Cascading" style definitions so we can have global settings and local overrides
• Save per-item specific style with the GUI params
• Have a different default object fill that is light but not white

[bgribble]

Here's a new color scheme courtesy of coolors:

[bgribble]

Closed by #273