Implement compatibility commands ef-themes-select-{dark,light}

We used to have those and I thought they were back in after the
redesign on top of Modus. But I never did that until the latest
commits to modus-themes.git.

This:

    commit a472f546459a80f3c95255118831d35a6dcbddf0
    Author: Protesilaos Stavrou <info@protesilaos.com>
    Date:   Sat Nov 22 16:47:08 2025 +0200

      Update modus-themes-define-derivative-command-known-suffixes to include select-{dark,light}

And this:

    commit c270cd8f5ea431534b5e9d6455239d890d9ed055
    Author: Protesilaos Stavrou <info@protesilaos.com>
    Date:   Sat Nov 22 14:14:49 2025 +0200

      Implement convenience commands modus-themes-select-{dark,light}

     doc/modus-themes.info | 348 +++++++++++++++++++++++++++-----------------------
     doc/modus-themes.org  |  25 +++-
     modus-themes.el       |  54 ++++++--
     3 files changed, 250 insertions(+), 177 deletions(-)

1 files changed, 6 insertions, 0 deletions

diff --git a/README.org b/README.org
index 9121e41..782408a 100644
--- a/README.org
+++ b/README.org
@@ -189,6 +189,12 @@ thus convenience wrappers around the equivalent Modus commands.
 #+findex: ef-themes-select
 - ~ef-themes-select~
 
+#+findex: ef-themes-select-dark
+- ~ef-themes-select-dark~ [ Part of {{{development-version}}}. ]
+
+#+findex: ef-themes-select-light
+- ~ef-themes-select-light~ [ Part of {{{development-version}}}. ]
+
 #+findex: ef-themes-load-random
 - ~ef-themes-load-random~