Perfect Darkness od Zabooby

If you want to tweak the colours, copy these colours into the Firefox Color add on https://cup.aihedy.com/en-GB/firefox/addon/firefox-color

"toolbar": "rgb(30, 32, 33)",
"toolbar_text": "rgb(255, 255, 255)",
"frame": "rgb(30, 32, 33)",
"tab_background_text": "rgb(255, 255, 255)",
"toolbar_field": "rgb(30, 32, 33)",
"toolbar_field_text": "rgb(255, 255, 255)",
"tab_line": "rgb(0, 0, 0)",
"popup": "rgb(30, 32, 33)",
"popup_text": "rgb(255, 255, 255)",
"icons": "rgb(255, 255, 255)",
"popup_border": "rgb(0, 0, 0)",
"popup_highlight_text": "rgb(255, 255, 255)",
"sidebar_border": "rgb(0, 0, 0)",
"sidebar_highlight_text": "rgb(255, 255, 255)",
"sidebar_text": "rgb(255, 255, 255)",
"sidebar": "rgb(30, 32, 33)",
"tab_loading": "rgb(0, 0, 0)",
"tab_text": "rgb(255, 255, 255)",
"toolbar_bottom_separator": "rgb(0, 0, 0)",
"toolbar_field_border_focus": "rgb(0, 0, 0)",
"toolbar_field_border": "rgb(30, 32, 33)",
"toolbar_field_focus": "rgb(30, 32, 33)",
"toolbar_field_highlight_text": "rgb(255, 255, 255)",
"toolbar_field_separator": "rgb(0, 0, 0)",
"toolbar_top_separator": "rgb(0, 0, 0)"