diff --git a/lib/outputlib.php b/lib/outputlib.php
index cda39fbd505..5c287e50907 100644
--- a/lib/outputlib.php
+++ b/lib/outputlib.php
@@ -716,7 +716,13 @@ class theme_config {
             $baseurl = new moodle_url($CFG->httpswwwroot.'/theme/styles_debug.php');
 
             if (!empty($this->lessfile)) {
-                $urls[] = new moodle_url($baseurl, array('theme' => $this->name, 'type' => 'less'));
+                if (core_useragent::is_ie() && !core_useragent::check_ie_version(10)) {
+                    // As the LESS file can get quite bit, and IE < 10 has troubles with files containing more than
+                    // 4095 selectors, we need to ask styles_debug.php to chunk the file.
+                    $urls[] = new moodle_url($baseurl, array('theme' => $this->name, 'type' => 'less', 'chunk' => 0));
+                } else {
+                    $urls[] = new moodle_url($baseurl, array('theme' => $this->name, 'type' => 'less'));
+                }
             } else {
                 $css = $this->get_css_files(true);
                 if (!$svg) {
diff --git a/theme/styles_debug.php b/theme/styles_debug.php
index 4e363ca6a52..33bef33ac6b 100644
--- a/theme/styles_debug.php
+++ b/theme/styles_debug.php
@@ -36,6 +36,7 @@ $type      = optional_param('type', '', PARAM_SAFEDIR);
 $subtype   = optional_param('subtype', '', PARAM_SAFEDIR);
 $sheet     = optional_param('sheet', '', PARAM_SAFEDIR);
 $usesvg    = optional_param('svg', 1, PARAM_BOOL);
+$chunk     = optional_param('chunk', null, PARAM_INT);
 
 if (file_exists("$CFG->dirroot/theme/$themename/config.php")) {
     // The theme exists in standard location - ok.
@@ -53,6 +54,9 @@ if ($type === 'editor') {
     css_send_uncached_css($csscontent);
 }
 
+$chunkurl = new moodle_url($CFG->httpswwwroot . '/theme/styles_debug.php', array('theme' => $themename,
+    'type' => $type, 'subtype' => $subtype, 'sheet' => $sheet, 'usesvg' => $usesvg));
+
 // We need some kind of caching here because otherwise the page navigation becomes
 // way too slow in theme designer mode. Feel free to create full cache definition later...
 $key = "$type $subtype $sheet $usesvg";
@@ -60,12 +64,27 @@ $cache = cache::make_from_params(cache_store::MODE_APPLICATION, 'core', 'themede
 if ($content = $cache->get($key)) {
     if ($content['created'] > time() - THEME_DESIGNER_CACHE_LIFETIME) {
         $csscontent = $content['data'];
+
+        // We need to chunk the content.
+        if ($chunk !== null) {
+            $chunks = css_chunk_by_selector_count($csscontent, $chunkurl->out(false));
+            $csscontent = ($chunk === 0) ? end($chunks) : $chunks[$chunk - 1];
+        }
+
         css_send_uncached_css($csscontent);
     }
 }
 
 $csscontent = $theme->get_css_content_debug($type, $subtype, $sheet);
-
 $cache->set($key, array('data' => $csscontent, 'created' => time()));
 
+// We need to chunk the content.
+if ($chunk !== null) {
+    // The chunks are ordered so that the last chunk is the one containing the @import, and so
+    // the first one to be included. All the other chunks are set in the array before that one.
+    // See {@link css_chunk_by_selector_count()} for more details.
+    $chunks = css_chunk_by_selector_count($csscontent, $chunkurl->out(false));
+    $csscontent = ($chunk === 0) ? end($chunks) : $chunks[$chunk - 1];
+}
+
 css_send_uncached_css($csscontent);