When using the include directive in kitty.conf make the environment variable KITTY_OS available for OS specific config

docs/changelog.rst

@@ -61,6 +61,7 @@ Detailed list of changes
 
 - When drawing the tab bar have the default left and right margins drawn in a color matching the neighboring tab (:iss:`5719`)
 
+- When using the :code:`include` directive in :file:`kitty.conf` make the environment variable :envvar:`KITTY_OS` available for OS specific config.
 
 0.26.5 [2022-11-07]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

docs/conf.rst

@@ -31,7 +31,9 @@ You can include secondary config files via the :code:`include` directive. If
 you use a relative path for :code:`include`, it is resolved with respect to the
 location of the current config file. Note that environment variables are
 expanded, so :code:`${USER}.conf` becomes :file:`name.conf` if
-:code:`USER=name`. Also, you can use :code:`globinclude` to include files
+:code:`USER=name`. A special environment variable :envvar:`KITTY_OS` is available,
+to detect the operating system. It is ``linux``, ``macos`` or ``bsd``.
+Also, you can use :code:`globinclude` to include files
 matching a shell glob pattern and :code:`envinclude` to include configuration
 from environment variables. For example::
 

docs/glossary.rst

@@ -212,3 +212,8 @@ Variables that kitty sets when running child programs
 
    Set when enabling :ref:`shell_integration` with :program:`bash`, allowing
    :program:`bash` to automatically load the integration script.
+
+.. envvar:: KITTY_OS
+
+   Set when using the include directive in kitty.conf. Can take values:
+   ``linux``, ``macos``, ``darwin``.