Add additional screen variants

This patch adds support for the following terminal variants in all four
color schemes:

  screen-16color
  screen-16color-bce
  screen-16color-s
  screen-16color-bce-s
  screen-256color-bce-s

Adding these allows this theme to be used with terminals that require the xterm-16color terminal type to be specified. (This includes my personal tmux and mobaXterm setup.)