core: replace darkgray color as default value of options

New default value is now used for these options:
- weechat.color.chat_inactive_buffer: "default"
- weechat.color.chat_inactive_window: "default"
- weechat.color.chat_nick_offline: "default"
- weechat.color.chat_nick_offline_highlight_bg: "blue"
- weechat.color.chat_prefix_buffer_inactive_buffer: "default"
- irc.color.topic_old: "default"
- logger.color.backlog_end: "default"
- logger.color.backlog_line: "default"
- script.color.text_delimiters: "default"

src/plugins/irc/irc-config.c

@@ -2701,7 +2701,7 @@ irc_config_init ()
         irc_config_file, ptr_section,
         "topic_old", "color",
         N_("color for old channel topic (when topic is changed)"),
-        NULL, -1, 0, "darkgray", NULL, 0, NULL, NULL,
+        NULL, -1, 0, "default", NULL, 0, NULL, NULL,
         NULL, NULL, NULL, NULL);
 
     /* network */

src/plugins/logger/logger-config.c

@@ -401,12 +401,12 @@ logger_config_init ()
         logger_config_file, ptr_section,
         "backlog_end", "color",
         N_("color for line ending the backlog"),
-        NULL, -1, 0, "darkgray", NULL, 0, NULL, NULL, NULL, NULL, NULL, NULL);
+        NULL, -1, 0, "default", NULL, 0, NULL, NULL, NULL, NULL, NULL, NULL);
     logger_config_color_backlog_line = weechat_config_new_option (
         logger_config_file, ptr_section,
         "backlog_line", "color",
         N_("color for backlog lines"),
-        NULL, -1, 0, "darkgray", NULL, 0, NULL, NULL, NULL, NULL, NULL, NULL);
+        NULL, -1, 0, "default", NULL, 0, NULL, NULL, NULL, NULL, NULL, NULL);
 
     /* file */
     ptr_section = weechat_config_new_section (logger_config_file, "file",

src/plugins/script/script-config.c

@@ -578,7 +578,7 @@ script_config_init ()
         script_config_file, ptr_section,
         "text_delimiters", "color",
         N_("text color of delimiters in script buffer"),
-        NULL, 0, 0, "darkgray", NULL, 0,
+        NULL, 0, 0, "default", NULL, 0,
         NULL, NULL, &script_config_refresh_cb, NULL, NULL, NULL);
     script_config_color_text_description = weechat_config_new_option (
         script_config_file, ptr_section,