Apply non-user themes only when asked

Theme settings now generally aren't actually applied until a call to
`enable-theme-, either one made explicitly or implicitly through
`load-theme' with NO-ENABLE nil.  This change has the effect of not
applying theme changes just because we load a lisp file containing a
theme specification.  The previous behavior is preserved for the
special case of the `user' theme, which is frequently used for
ad-hoc customization.

* lisp/cus-face.el (custom-theme-set-faces): Call
`custom--should-apply-setting' to decide whether to apply
a setting.

* lisp/custom.el (custom--should-apply-setting): New function.
(custom--inhibit-theme-enable): Add `apply-only-user' option;
default to it.
(custom-push-theme, custom-theme-set-variables): Call
`custom--should-apply-setting' to decide whether to apply
a setting.

1 files changed, 9 insertions, 0 deletions

diff --git a/etc/NEWS b/etc/NEWS
index d59b4a7cf4f..39b8b5ca0c8 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -518,6 +518,15 @@ names" in the Tramp manual for full documentation of these facilities.
 
 * Incompatible Lisp Changes in Emacs 27.1
 
++++
+** Theme settings generally aren't actually applied until a call to
+`enable-theme-, either one made explicitly or implicitly through
+`load-theme' with NO-ENABLE nil.  This change has the effect of not
+applying theme changes just because we load a lisp file containing a
+theme specification.  The previous behavior is preserved for the
+special case of the `user' theme, which is frequently used for
+ad-hoc customization.
+
 ** The 'repetitions' argument of 'benchmark-run' can now also be a variable.
 ** The FILENAME argument to 'file-name-base' is now mandatory and no
 longer defaults to 'buffer-file-name'.