feat(ui-theme): default the theme fallback to bundled dupre

The fallback kicks in when persist/emacs-theme is missing — a fresh machine, or one that's never saved a theme. It was modus-vivendi, which ships with Emacs but has none of the dimming colors this config chooses, so an unconfigured machine looked and dimmed differently from a configured one. I hit exactly that on a second box this week.

dupre is bundled in themes/ and carries those colors, and it loads wherever this config does, so it's the better default. I added a regression test asserting the default is dupre; its loadability is already covered by test-dupre-theme.el. The docstring no longer claims the fallback must be a built-in theme, since dupre isn't one.

1 files changed, 6 insertions, 3 deletions

diff --git a/modules/ui-theme.el b/modules/ui-theme.el
index 2df37000..a7873b9a 100644
--- a/modules/ui-theme.el
+++ b/modules/ui-theme.el
@@ -64,11 +64,14 @@ directory that is sync'd across machines with this configuration."
   :type 'file
   :group 'cj/ui-theme)
 
-(defcustom fallback-theme-name "modus-vivendi"
+(defcustom fallback-theme-name "dupre"
   "The name of the theme to fallback on.
 This is used when there's no file, or the theme name doesn't match
-any of the installed themes. This should be a built-in theme. If theme name is
-`nil', there will be no theme."
+any of the installed themes. It must be available wherever this config is
+loaded, since the fallback has no further fallback. dupre is bundled in
+themes/ and carries the dimming colors chosen for this config, so it is the
+default; a built-in theme like modus-vivendi works too but has no chosen
+dimming colors. If theme name is `nil', there will be no theme."
   :type 'string
   :group 'cj/ui-theme)