add support for an override stylesheet which applies to all users

include/functions.php

@@ -7,6 +7,10 @@
 
 	define('COOKIE_LIFETIME_LONG', 86400*365);
 
+	// this CSS file is included for everyone (if it exists in themes.local)
+	// on login, registration, and main (index and prefs) pages
+	define('LOCAL_OVERRIDE_STYLESHEET', '.local-overrides.css');
+
 	$fetch_last_error = false;
 	$fetch_last_error_code = false;
 	$fetch_last_content_type = false;

include/login_form.php

@@ -18,6 +18,10 @@
 
 	} ?>
 
+	<?php if (theme_exists(LOCAL_OVERRIDE_STYLESHEET)) {
+		echo stylesheet_tag(get_theme_path(LOCAL_OVERRIDE_STYLESHEET));
+	} ?>
+
 	<style type="text/css">
 		@media (prefers-color-scheme: dark) {
 			body {