As far as I can tell, the theme does not set the Terminal highlight group, which for me means a :terminal window has default foreground and background colours.

I would expect something like this does the trick:

"+- Vim 8 Terminal Colors -+
if has('terminal')
  let g:terminal_ansi_colors = [...]
  call s:hi("Terminal", ...)
endif

Relevant part of :h terminal-size-color:

To use a different color the Terminal highlight group can be used, for example:
hi Terminal ctermbg=lightgrey ctermfg=blue guibg=lightgrey guifg=blue