core: add core-theme skeleton and theme registry

Introduce a new module (core-theme.{c,h}) holding the in-memory registry
of built-in themes used by the upcoming /theme command:

- struct t_theme stores name, description, date and weechat version
  captured at registration time, plus a hashtable of overrides keyed by
  full option name (file.section.option) -> value string.
- theme_register (name, overrides) creates a new theme or merges the
  given overrides into an existing one (later calls override duplicate
  keys); this is the API plugins and scripts will use to contribute
  per-theme color values.
- theme_search and theme_list provide lookup and ordered enumeration.
- theme_init / theme_end are called from weechat_init / weechat_end.

The theme_applying flag is declared here but not yet consumed (it will
gate config_change_color in the next commit to avoid N redundant
window refreshes during /theme apply).

User theme files are not handled by this module: they are read
transiently inside /theme apply (a later commit) and never cached.

src/core/CMakeLists.txt

@@ -51,6 +51,7 @@ set(LIB_CORE_SRC
   core-signal.c core-signal.h
   core-string.c core-string.h
   core-sys.c core-sys.h
+  core-theme.c core-theme.h
   core-upgrade.c core-upgrade.h
   core-upgrade-file.c core-upgrade-file.h
   core-url.c core-url.h

src/core/core-theme.h

@@ -0,0 +1,51 @@
+/*
+ * SPDX-FileCopyrightText: 2026 Sébastien Helleu <flashcode@flashtux.org>
+ *
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ *
+ * This file is part of WeeChat, the extensible chat client.
+ *
+ * WeeChat is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * WeeChat is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with WeeChat.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+#ifndef WEECHAT_THEME_H
+#define WEECHAT_THEME_H
+
+struct t_hashtable;
+struct t_arraylist;
+
+struct t_theme
+{
+    char *name;                           /* "dark", "solarized", ...        */
+    char *description;                    /* free-form text                  */
+    char *date;                           /* "YYYY-MM-DD HH:MM:SS"           */
+    char *weechat_version;                /* version at registration time    */
+    struct t_hashtable *overrides;        /* full_option_name -> value       */
+    struct t_theme *prev_theme;           /* link to previous theme          */
+    struct t_theme *next_theme;           /* link to next theme              */
+};
+
+extern struct t_theme *themes;
+extern struct t_theme *last_theme;
+extern int theme_applying;                /* gate for config_change_color    */
+
+extern struct t_theme *theme_search (const char *name);
+extern struct t_theme *theme_register (const char *name,
+                                       struct t_hashtable *overrides);
+extern struct t_arraylist *theme_list (void);
+
+extern void theme_init (void);
+extern void theme_end (void);
+
+#endif /* WEECHAT_THEME_H */

src/core/weechat.c

@@ -75,6 +75,7 @@
 #include "core-secure-config.h"
 #include "core-signal.h"
 #include "core-string.h"
+#include "core-theme.h"
 #include "core-upgrade.h"
 #include "core-url.h"
 #include "core-utf8.h"
@@ -384,6 +385,7 @@ weechat_init (int argc, char *argv[], void (*gui_init_cb)(void))
         weechat_shutdown (EXIT_FAILURE, 0);
     if (!secure_config_init ())         /* init secured data options (sec.*)*/
         weechat_shutdown (EXIT_FAILURE, 0);
+    theme_init ();                      /* initialize theme registry        */
     if (!config_weechat_init ())        /* init WeeChat options (weechat.*) */
         weechat_shutdown (EXIT_FAILURE, 0);
     args_parse (argc, argv);            /* parse command line args          */
@@ -449,6 +451,7 @@ weechat_end (void (*gui_end_cb)(int clean_exit))
     unhook_all ();                      /* remove all hooks                 */
     hdata_end ();                       /* end hdata                        */
     secure_end ();                      /* end secured data                 */
+    theme_end ();                       /* end theme registry               */
     string_end ();                      /* end string                       */
     weeurl_end ();
     weechat_shutdown (-1, 0);           /* end other things                 */

tests/unit/CMakeLists.txt

@@ -75,6 +75,7 @@ set(LIB_WEECHAT_UNIT_TESTS_CORE_SRC
   core/test-core-secure.cpp
   core/test-core-signal.cpp
   core/test-core-string.cpp
+  core/test-core-theme.cpp
   core/test-core-url.cpp
   core/test-core-utf8.cpp
   core/test-core-util.cpp

tests/unit/tests.cpp

@@ -77,6 +77,7 @@ IMPORT_TEST_GROUP(CoreNetwork);
 IMPORT_TEST_GROUP(CoreSecure);
 IMPORT_TEST_GROUP(CoreSignal);
 IMPORT_TEST_GROUP(CoreString);
+IMPORT_TEST_GROUP(CoreTheme);
 IMPORT_TEST_GROUP(CoreUrl);
 IMPORT_TEST_GROUP(CoreUtf8);
 IMPORT_TEST_GROUP(CoreUtil);