Add a GTK4 / libadwaita theme

[george-petrakis]

Adds a GTK4 / libadwaita stylesheet to Chicago95. This addresses #256 ("Theme not applying to GTK-4") and covers the GTK4 part of #363 ("use this on gnome?").

At the moment every GTK4 app is unthemed, and on a current GNOME that covers most of the desktop, since Files, Settings, Text Editor and Console are all libadwaita.

Why the earlier GTK4 attempts didn't stick

The prototype in #256 was abandoned in 2024. The reason matters, because it dictates how this has to be shipped.

libadwaita ignores gtk-theme. On adw_init() it sets gtk-theme-name to Adwaita-empty, which is a real stylesheet shipped inside libgtk that contains zero bytes, and then installs its own three providers at GTK_STYLE_PROVIDER_PRIORITY_THEME (200). The result is that ~/.themes/Chicago95/gtk-4.0/gtk.css does get found and parsed, GTK even warns about loading it, and then it is discarded. A theme that ships only a theme directory is a no-op in every libadwaita app while looking like it should work. I confirmed this at runtime: with the theme installed, gtk-theme-name reads back as Adwaita-empty.

The hook that does survive is $XDG_CONFIG_HOME/gtk-4.0/gtk.css. GTK loads that at PRIORITY_USER (800), which is above libadwaita's own sheet, so that is where the theme has to go. installer.py now installs it there.

GTK_THEME=Chicago95 is not the recommended route. It does change the look, but only because it makes libadwaita skip loading its stylesheet altogether, so its widgets come out unstyled rather than restyled.

What's here

Theme/Chicago95/gtk-4.0/, split the same way gtk-3.0 already is, with gtk.css holding the palette, the global resets and the @import list:

file	covers
gtk.css	palette, libadwaita colour overrides, resets, imports
gtk-widgets.css	button, entry, check/radio, switch, spinbutton, scale, progressbar
gtk-containers.css	window/CSD titlebar, windowcontrols, notebook, scrollbar, paned
gtk-lists.css	listview, columnview, gridview, treeexpander, sidebars
gtk-menus.css	popovers, menus, dropdowns, tooltips, dialogs, toasts
gtk-adwaita.css	AdwToolbarView, preferences rows, .boxed-list, tabs, status pages
gtk-dark.css	dark-variant entry point; Chicago95 has one look, so it re-imports gtk.css
settings.ini	theme-level GTK settings

Palette and metrics come from gtk-3.0 rather than being re-picked, including the parts that look like typos but aren't. bg_bright is shade(white, 0.99), which computes to #fcfcfc rather than #ffffff, and the GTK4 sheet uses the same value. There is no border-radius, and no gradients, shadows or transitions.

gtk-dark.css is not filler. With color-scheme=prefer-dark GTK asks for the dark variant, and when it is missing GTK falls back to its own builtin Default-dark rather than to gtk.css, so Chicago95 would disappear entirely for anyone running dark mode.

Also in this branch:

• installer.py installs the user CSS inside a marker-delimited block, keeps a backup, and gains disable_gtk4_user_css() to remove it again. It also gains a GNOME branch that configures the theme with gsettings instead of xfconf-query, chosen from XDG_CURRENT_DESKTOP rather than assuming Xfce.
• INSTALL.md gets a GNOME/GTK4 section that spells out what this does not cover.

Limits

• GNOME Shell is not themed. The in-tree gnome-shell/ theme targets 3.32, and 46% of its selectors no longer exist on Shell 50. That is out of scope here.
• Snap and Flatpak apps are not covered, since they don't see the host's user CSS.
• Window decorations are CSD under Mutter, so the xfwm4 themes don't apply. The CSD titlebar is styled instead, including headerbar.default-decoration, which is what mutter-x11-frames uses for Xwayland windows.
• No thumbnail.png. A maintainer screenshot would be better than anything I could make up.
• Window buttons use symbolic icons recoloured to black, so they follow whatever icon theme is active. With the Chicago95 icon theme they look right. The sheet does not force -gtk-icon-style: regular, because that makes GTK substitute whatever full-colour legacy icon the active theme ships; under Yaru that turns the close button into a red circle.

Verification

Checked on GTK 4.22.4 / libadwaita 1.9.1 (Ubuntu 26.04, GNOME 50, Wayland):

• the sheet parses through GtkCssProvider with 0 errors, with @imports followed;
• the Chicago95 palette resolves in both a libadwaita process and a plain GTK4 one, checked by reading the resolved values back rather than by eye;
• gtk4-widget-factory and adwaita-1-demo both run with no GTK warnings. An earlier revision of this branch produced 14 negative-min-size warnings; those are fixed, with the causes written up in comments next to the rules;
• all 44 relative url() asset references resolve;
• no undeclared @colour references.

Not checked: GNOME Shell, snaps and flatpaks, HiDPI, RTL, and Xfce. I don't have an Xfce session to confirm the GTK3 side still behaves, though no gtk-3.0 file is touched by this branch.

If you'd rather the installer changes landed separately, or want the files named or split differently, I'm glad to rework it.