I hope this ok for a solved issue.
GIMP Pitch Black Theme (update-safe, independent of GTK theming)
This creates a custom GIMP theme in your home directory.
1. Create the theme folder
mkdir -p ~/.config/GIMP/3.0/themes/PitchBlack
2. Create the CSS file
Create ~/.config/GIMP/3.0/themes/PitchBlack/gimp-dark.css with the contents below.
/* Pitch Black variant — adapted from GIMP 3.0 Dark default */
/* Basic foreground, background and border colors. */
@define-color fg-color rgb(210,210,210);
@define-color bg-color rgb(16,16,16);
@define-color border-color rgb(4,4,4);
/********* Variants for foreground colors *********/
@define-color dimmed-fg-color rgb(115,115,115);
@define-color disabled-fg-color rgb(95,95,95);
@define-color disabled-button-color rgb(65,65,65);
/********* Variants for background colors *********/
/* Hover needs to be visibly lighter than bg on a near-black base */
@define-color hover-color rgb(38,38,38);
/* widget-bg slightly darker than bg so it recedes */
@define-color widget-bg-color rgb(10,10,10);
/* selected pops against widget-bg */
@define-color selected-color rgb(28,28,28);
/* extreme-bg is the absolute darkest surface */
@define-color extreme-bg-color rgb(6,6,6);
/* extreme-selected must still be distinguishable from extreme-bg */
@define-color extreme-selected-color rgb(22,22,22);
/********* Variants for border colors *********/
@define-color strong-border-color rgb(12,12,12);
@define-color stronger-border-color rgb(32,32,32);
@define-color edge-border-color rgb(3,3,3);
/********* Colors for special cases *********/
@define-color scrollbar-slider-color rgb(72,72,72);
@define-color scrollbar-trough-color rgb(12,12,12);
@define-color ruler-color rgba(8,8,8,0.5);
@import url("/usr/share/gimp/3.0/themes/Default/common-dark.css");
Note the import path — because this file lives outside the Default theme folder, the
@importmust use the full absolute path.
Check if path exist before proceeding.
l /usr/share/gimp/3.0/themes/Default/common-dark.css
3. Apply in GIMP
Edit → Preferences → Interface → Theme: PitchBlack / Color scheme: Dark
Restart GIMP if it was already open.
NOTE:
For some reason when I use the .config css it is slidely greyer.
The first time I tested I changed the original /user/share../gimp-dark.css and the look was picth-black.
The downside of editing the default theming is it will change after a up[date of gimp and one need to do the steps again.
Have fun 