Implement CSS theming

[stephenmparke]

Priority implementation:

• Better default theme (rather than colors from thin air)
• Dark theme

Secondary implementation:

• pre-defined color presets

Pie-in-the-sky:

• User-defined color sets

[stephenmparke]

Attaching first pass at a default color palette. This is definitely open for discussion, but gives a decent starting point.

pallete-split.zip