Set themes even if detection mechanism fails

Previously, failing to “determine a viable theme detection mechanism” would
error, preventing the rest of `auto-dark-mode` setup from running. This is now a
warning, and you are effectively left in “manual” mode.

This is technically a fix for LionyxML#66, but doesn’t address all the related changes
there. Those will be addressed in LionyxML#62. It also partially addresses LionyxML#73 by adding
`auto-dark-toggle-appearance`.

auto-dark.el

@@ -238,6 +238,19 @@ This will load themes if necessary."
         (warn "Failed to enable theme(s): %s"
               (mapconcat #'symbol-name failures ", "))))))
 
+(defun auto-dark-toggle-appearance ()
+  "Switch between light and dark mode.
+If `auto-dark-detection-method' is nil, this will persist until the next time
+this is called. Otherwise, it could switch to the system appearance at any
+time."
+  (interactive)
+  (auto-dark--set-theme (if (eq auto-dark--last-dark-mode-state 'dark)
+                            'light
+                          ;; NB: This does _something_ even if we don’t know
+                          ;;     what the previous state was, since the user
+                          ;;     explicitly requested a change.
+                          'dark)))
+
 (defun auto-dark--set-theme (appearance)
   "Set light/dark theme Argument APPEARANCE should be light or dark."
   (setq auto-dark--last-dark-mode-state appearance)
@@ -346,7 +359,9 @@ Remove theme change callback registered with D-Bus."
    ((eq system-type 'windows-nt)
     'winreg)
    (t
-    (error "Could not determine a viable theme detection mechanism!"))))
+    (lwarn 'auto-dark :error "Could not determine a viable theme detection \
+mechanism! You can use ‘auto-dark-toggle-appearance’ to manually switch between \
+modes."))))
 
 ;;;###autoload
 (define-minor-mode auto-dark-mode

tests/basic.el

@@ -42,6 +42,16 @@
     (expect auto-dark-themes :to-equal '((wombat) (leuven)))
     (expect custom-enabled-themes :to-equal '(leuven))))
 
+(describe "toggling"
+  (before-each
+    (auto-dark-toggle-appearance))
+  (it "should invert the current appearance"
+    (expect custom-enabled-themes :to-equal '(wombat)))
+  (it "should invert the current appearance again"
+    (expect custom-enabled-themes :to-equal '(leuven)))
+  (it "should invert the current appearance and again"
+    (expect custom-enabled-themes :to-equal '(wombat))))
+
 ;; Restore the original state
 (unless auto-dark-tests--original-state
   (auto-dark-mode -1))