feat(grub): render grub-theme-udt's theme.txt and print its root install

theme.txt has no include, so udt-palette substitutes eight roles and the
scheme name into the sibling repo's theme.txt.in, conky-style. The accent is
the scheme's fixed one; nothing at boot can read the wallpaper.

install.sh runs that repo's build as the user and, only when it succeeds,
prints the root copy into /boot/grub/themes plus the one-time GRUB_THEME and
grub-mkconfig steps. External targets are now skipped whenever their template
is missing, not only when the repo is, so a sibling mid-setup cannot break an
install.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

3 files changed, 41 insertions, 3 deletions

diff --git a/AGENTS.md b/AGENTS.md
index 3acdc98..df16d3d 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -23,6 +23,10 @@ Sibling checkouts are wired from here, each skipped when not cloned:
 `waybar-theme-udt` (its own `install.sh`), `conky-theme-udt` (five links into
 `~/.config/conky/`) and `hypr-theme`, which is the whole of `~/.config/hypr` as
 one symlink. A real `~/.config/hypr` directory is left alone with a warning.
+`grub-theme-udt` is SDDM's shape: udt-palette renders its `theme.txt.in`,
+`install.sh` runs its `build` as the user, then prints the root copy. A copy,
+not a link, because GRUB reads `/boot` before `/home` is mounted; it needs
+rerunning after a scheme or wallpaper change.
 
 Live files that mix theme keys with anything else are merged, never replaced:
 `install.sh`'s `ini_merge` writes every key in `templates/qt-gtk/qt5ct.conf`,
diff --git a/bin/udt-palette b/bin/udt-palette
index 6a15a52..00e3092 100755
--- a/bin/udt-palette
+++ b/bin/udt-palette
@@ -30,6 +30,7 @@ SELECTOR_SEED = PALETTE_DIR / "roles.conf.default"
 # The Lua dashboard lives in its own repo; UDT only renders its colours.
 # A sibling checkout, like waybar-theme-udt; skipped when it is not cloned.
 CONKY_REPO = REPO.parent / "conky-theme-udt"
+GRUB_REPO = REPO.parent / "grub-theme-udt"
 
 
 def parse_conf(path):
@@ -688,6 +689,20 @@ def gen_conky(res, scheme, palette, template):
     return out
 
 
+def gen_grub(res, scheme, palette, template):
+    """GRUB: substitute into the theme template in the grub-theme-udt repo.
+
+    theme.txt has no include, so the colours are baked in. The accent is the
+    scheme's fixed one: nothing at boot can read the wallpaper. The PNG slices
+    that carry colour are drawn by that repo's build script from this output.
+    """
+    out = template.replace("@SCHEME@", scheme)
+    for role in ("bg", "bg_alt", "bg_deep", "surface", "fg", "fg_dim",
+                 "fg_faint", "accent"):
+        out = out.replace(f"@{role.upper()}@", hex6(res[role]))
+    return out
+
+
 def gen_accent_py(res, scheme, palette, snap_names):
     """The accent table and Macchiato dict udt-accent carries for Firefox.
 
@@ -787,6 +802,7 @@ TARGETS = [
     ("templates/hyprlock/hyprlock-palette.conf", gen_hyprlock, None),
     ("templates/nvim/udt.lua", gen_nvim, None),
     (str(CONKY_REPO / "conky.conf"), gen_conky, str(CONKY_REPO / "conky.conf.in")),
+    (str(GRUB_REPO / "theme.txt"), gen_grub, str(GRUB_REPO / "theme.txt.in")),
     ("bin/udt_colors.py", gen_accent_py, None),
     ("templates/homepage/custom.css", gen_homepage, None),
     ("templates/obsidian/udt.css", gen_obsidian, None),
@@ -817,9 +833,9 @@ def generate(roles_path, out_dir, scheme=None):
     scheme, palette, res, snap_names = load(roles_path, scheme=scheme)
     written = []
     for target, fn, template in TARGETS:
-        # A sibling repo that is not cloned here is not an error: its
-        # template is missing too, and there is nothing to theme.
-        if Path(target).is_absolute() and not Path(target).parent.is_dir():
+        # A sibling repo that is not cloned here, or has no template yet, is
+        # not an error: there is nothing to theme.
+        if Path(target).is_absolute() and not (template and Path(template).exists()):
             continue
         # A target outside this repo (the conky dashboard) is an absolute path,
         # and `out_dir / absolute` discards out_dir entirely. For the real
diff --git a/install.sh b/install.sh
index 9626431..abcfd51 100755
--- a/install.sh
+++ b/install.sh
@@ -339,6 +339,24 @@ if [ -d "$sddm_repo/theme" ]; then
     echo "  printf '[Theme]\\nCurrent=udt\\n' > /etc/sddm.conf.d/udt.conf"
 fi
 
+# GRUB. Same shape as SDDM: udt-palette has rendered theme.txt, the sibling
+# repo's build assembles the theme as the user, and the root half is printed.
+# A copy, not a link: GRUB reads /boot before /home is mounted. The background
+# is a snapshot of the wallpaper, so the copy needs rerunning after a scheme
+# or wallpaper change; grub-mkconfig only when GRUB_THEME changes.
+grub_repo="$(realpath -m "$repo/../grub-theme-udt")"
+if [ -x "$grub_repo/build" ]; then
+    if "$grub_repo/build"; then
+        echo
+        echo "grub theme: run these as root to install or refresh it"
+        echo "  rm -rf /boot/grub/themes/udt && cp -r \"$grub_repo/build/udt\" /boot/grub/themes/"
+        echo "  # once: GRUB_THEME=\"/boot/grub/themes/udt/theme.txt\" in /etc/default/grub, then"
+        echo "  grub-mkconfig -o /boot/grub/grub.cfg"
+    else
+        echo "grub: its build failed, theme not refreshed" >&2
+    fi
+fi
+
 [ -n "$reloaded" ] && echo "reloaded:$reloaded"
 
 echo "installed:"