add default theme to dark

js/mastodon-config.php

@@ -12,6 +12,7 @@ window.addEventListener("load", () => {
     btnSeeMore: "<?php echo MASTODON_BTN_SEE_MORE; ?>",
     btnReload: "<?php echo MASTODON_BTN_RELOAD; ?>",
     maxNbPostFetch: '<?php echo MASTODON_MAX_POST_FETCH; ?>',
-    maxNbPostShow: '<?php echo MASTODON_MAX_POST_SHOW; ?>'
+    maxNbPostShow: '<?php echo MASTODON_MAX_POST_SHOW; ?>',
+    defaultTheme: 'dark'
   });
 });