theme: support theme setting override

...by reading <config-dir>/themerc-override where <config-dir> is normally
$HOME/.config/labwc can be other locations as described in labwc-config(5)
and can also be specified by the command line option -C.

The reason for supporting theme override is to give users more fine-
grained control of settings without making local copies and modifying
themes.

docs/README

@@ -3,5 +3,7 @@ Config layout for ~/.config/labwc/
 - environment
 - menu.xml
 - rc.xml
+- themerc-override
+
+See `man labwc-config and `man labwc-theme` for further details.
 
-See man labwc-config for further details.

docs/labwc-config.5.scd

@@ -18,6 +18,9 @@ searched for in the following order:
 - ${XDG_CONFIG_HOME:-$HOME/.config}/labwc
 - ${XDG_CONFIG_DIRS:-/etc/xdg}/labwc
 
+The configuration directory location can be override with the -C command line
+option.
+
 All configuration and theme files except autostart are re-loaded on receiving
 signal SIGHUP.
 

docs/labwc-theme.5.scd

@@ -20,6 +20,11 @@ the rc.xml configuration file (labwc-config(5)).
 
 A theme consists of a themerc file and optionally some xbm icons.
 
+Theme settings specified in themerc can be overridden by creating a
+'themerc-override' file in the configuration directory, which is normally
+$HOME/.config/labwc/ but can be a few other locations as described in
+labwc-config(5).
+
 # DATA TYPES
 
 *color RGB values*

docs/meson.build

@@ -30,6 +30,7 @@ install_data(
     'environment',
     'menu.xml',
     'README',
+    'themerc',
     'rc.xml',
     'rc.xml.all'
   ],

docs/themerc

@@ -1,3 +1,10 @@
+# This file contains all themerc options with default values
+#
+# System-wide and local themes can be overridden by creating a copy of this
+# file and renaming it to $HOME/.config/labwc/themerc-override. Be careful
+# though - if you only want to override a small number of specific options,
+# make sure all other lines are commented out or deleted.
+
 # general
 border.width: 1
 padding.height: 3