diff --git a/.htaccess b/.htaccess
index 196e613a7..5671d98ba 100644
--- a/.htaccess
+++ b/.htaccess
@@ -42,6 +42,7 @@ RewriteRule ^~~([^/]*)/?$ /~~$1/lobby [R=301,L]
 RewriteRule ^~~([^/]*)/(teambuilder|ladder|lobby|battle)/?$ index.php?server=$1 [L,QSA]
 RewriteRule ^~~([^/]*)/(lobby|battle)-([A-Za-z0-9-]*)$ index.php?server=$1 [L,QSA]
 RewriteRule ^~~([^:/]*)(:[0-9]*)?/action\.php$ action.php?serverid=$1 [L,QSA]
+RewriteRule ^style/custom/([A-Za-z0-9-]*)\.css$ customcss.php?server=$1 [L]
 
 RewriteRule ^backup/ - [F]
 RewriteRule ^\.git/ - [F]
diff --git a/customcss.php b/customcss.php
new file mode 100644
index 000000000..0f7cfccf8
--- /dev/null
+++ b/customcss.php
@@ -0,0 +1,58 @@
+<?php
+
+include '../pokemonshowdown.com/config/servers.inc.php';
+
+header('Content-type: text/css');
+
+$server = @$_REQUEST['server'];
+$customcssuri = @$PokemonServers[$server]['customcss'];
+if (empty($customcssuri)) {
+	die();
+}
+
+$cssfile = '../pokemonshowdown.com/config/customcss/' . $server;
+
+// No need to sanitise $server because it should be safe already.
+$lastmodified = @filemtime($cssfile);
+if ($lastmodified && (time() - $lastmodified) < 3600) {
+	// Don't check for modifications more than once an hour.
+	readfile($cssfile);
+	die();
+}
+
+$curl = curl_init($customcssuri);
+if ($lastmodified) {
+	curl_setopt($curl, CURLOPT_HTTPHEADER, array(
+		'If-Modified-Since: ' . gmdate('D, d M Y H:i:s T', $lastmodified)
+	));
+}
+curl_setopt($curl, CURLOPT_FOLLOWLOCATION, true);
+curl_setopt($curl, CURLOPT_MAXREDIRS, 5);
+curl_setopt($curl, CURLOPT_RETURNTRANSFER, true);
+$curlret = curl_exec($curl);
+if ($curlret) {
+	$code = curl_getinfo($curl, CURLINFO_HTTP_CODE);
+	if ($code === 304) {
+		// No modifications.
+		readfile($cssfile);
+		touch($cssfile);	// Don't check again for an hour.
+	} else if ($code === 200) {
+		// Sanitise the CSS.
+		require '../pokemonshowdown.com/lib/htmlpurifier/HTMLPurifier.auto.php';
+		require '../pokemonshowdown.com/lib/csstidy/class.csstidy.php';
+		$config = HTMLPurifier_Config::createDefault();
+		$config->set('Filter.ExtractStyleBlocks', TRUE);
+		$purifier = new HTMLPurifier($config);
+		$level = error_reporting(E_ALL & ~E_STRICT);
+		$html = $purifier->purify('<style>' . $curlret . '</style>');
+		error_reporting($level);
+		list($outputcss) = $purifier->context->get('StyleBlocks');
+		file_put_contents($cssfile, $outputcss);
+		echo $outputcss;
+	} else {
+		// Error condition - clear the cached file.
+		file_put_contents($cssfile, '');
+	}
+}
+curl_close($curl);
+
diff --git a/index.php.template.txt b/index.php.template.txt
index 150178960..19f8a363e 100644
--- a/index.php.template.txt
+++ b/index.php.template.txt
@@ -107,7 +107,7 @@ if (!$challengeresponse) {
 		<link rel="stylesheet" href="/style/replayer.css" /><!-- utilichart -->
 		<link rel="stylesheet" href="/style/font-awesome.css" />
 <?php if (@$serverdata['customcss']) { ?>
-		<link rel="stylesheet" href="<?php echo $serverdata['customcss'] ?>" />
+		<link rel="stylesheet" href="/style/custom/<?php echo $serverid ?>.css" />
 <?php } ?>
 		<meta id="viewport" name="viewport" content="width=640"/>
 		<meta http-equiv="X-UA-Compatible" content="IE=Edge,chrome=1" />