How to set background color of lines in IndentLine?

Question

I'm trying to configure Yggdroot/indentLine to use the same background color as used in whatever color scheme I'm using, that is I'd like the gray parts to have the same color as the background. From various screenshots I assumed this somehow must be possible, but I couldn't find any option that would actually influence this color so far. Finally I'd also like to change the color of the characters themselves too, but nothing I tried influenced these either. (The options I played with commented out below.)

The ~/.vimrc I used here was this:

call plug#begin('~/.vim/plugged')
Plug 'Yggdroot/indentLine'
call plug#end()

"let g:indentLine_setColor = 0                  "doesn't seem to change anything
"let g:indentLine_defaultGroup = 'SpecialKey'   "doesn't seem to change anything
"let g:indentLine_color_term = 1                "doesn't seem to change anything
"let g:indentLine_color_term = 239              "doesn't seem to change anything
let g:indentLine_char_list = [' ', ' ', '.']    "works as expected

set expandtab
set tabstop=4
color default