Themes for xterm.js

[dhobsd]

Different terminals have different settings, and it can be odd for someone using a 16-color terminal to have their colors misinterpreted by something assuming xterm colors. Mostly this would just be styling colors to match various terminals. Then castty could detect what kind of terminal it was running in (using $TERM) and feed that info to the player, which could auto-choose the theme.

Themes related to accessibility would also be nice.