GIMP

GIMP has its own personal-override CSS hook, separate from the auto-generated theme.css that ships with the app (which is regenerated on every theme change and explicitly says not to edit it by hand). This template writes to that personal file directly: gimp.css, imported automatically if present, no extra setup needed once the template is enabled.

What gets themed, and the two things that needed extra care

GIMP's own theme only defines 17 named colors (fg-color, bg-color, selected-color, and so on), reused across common.css/common-dark.css. Most of the UI follows from setting those. Two things needed more than a plain 1:1 color swap:

Context menus and other GtkPopoverMenu content had no background rule at all in GIMP's own theme (only narrow button/list-row styling), so they fell through to GTK's stock light default. Fixed with explicit popover/popover contents rules.
selected-color/extreme-selected-color are always paired with white text in GIMP's own CSS, with no dark-text variant the way Noctalia's other templates use on_primary on top of a primary-colored background. Mapping them straight to Noctalia's yellow accent made text unreadable everywhere that yellow shows up (menu hovers, selected list rows, the Preferences sidebar). Fixed by keeping the yellow and adding an explicit dark-text override for every selector GIMP's common.css actually uses those two variables as a background for (checked via grep, not guessed).
A few widgets, confirmed just the Preferences dialog's own page title bar, don't use GIMP's custom variables at all. GIMP's own source comment on that exact element admits it "doesn't have any more specific way to style it," meaning it falls back to standard GTK/Adwaita variable names (@theme_bg_color, @theme_selected_bg_color, etc.) instead. Those are set too, as a safety net, since the system gtk.css those would normally come from never loads inside a Flatpak sandbox.

Requires GIMP's own "Default" base theme

GIMP has two selectable base themes (Preferences > Interface > Theme): Default and System. This template is built against Default (themes/Default/gimp-dark.css), the one this personal override's own 17 variables and extra selectors are checked against. System imports a different, much smaller CSS file with no guarantee it uses the same variable names, not tested or supported here. If colors look wrong after enabling this template, check Preferences first.

Native install

gimp.css lives at ~/.config/GIMP/3.2/gimp.css.

Flatpak install (`org.gimp.GIMP`)

Same file, inside the sandbox's own isolated config: ~/.var/app/org.gimp.GIMP/config/GIMP/3.2/ gimp.css. No flatpak override needed, GIMP reads its own config from inside its sandbox regardless of any host ~/.config access.

Testing

Tested against GIMP 3.2.4 (Flatpak). Verified visually across three passes: main window and toolbox (screenshot.png), right-click context menus, and the Preferences dialog specifically (screenshot-preferences.png), the hardest case since it touches nearly every one of GIMP's 17 variables plus the GTK fallback ones.

README.md