diff --git a/admin/tool/behat/cli/util_single_run.php b/admin/tool/behat/cli/util_single_run.php
index 967fde33358..f0a3650f540 100644
--- a/admin/tool/behat/cli/util_single_run.php
+++ b/admin/tool/behat/cli/util_single_run.php
@@ -164,6 +164,8 @@ if ($options['install']) {
         mtrace("Acceptance tests site installed");
     }
 
+    // Note: Do not build the themes here. This is done during the 'enable' stage.
+
 } else if ($options['drop']) {
     // Ensure no tests are running.
     test_lock::acquire('behat');
@@ -182,6 +184,10 @@ if ($options['install']) {
     // Enable test mode.
     behat_util::start_test_mode($options['add-core-features-to-theme'], $options['optimize-runs'], $parallel, $run);
 
+    // Themes are only built in the 'enable' command.
+    behat_util::build_themes();
+    mtrace("Testing environment themes built");
+
     // This is only displayed once for parallel install.
     if (empty($run)) {
         // Notify user that 2.5 profile has been converted to 3.5.
diff --git a/lib/behat/classes/util.php b/lib/behat/classes/util.php
index 267406bb5bf..fda4f1c1321 100644
--- a/lib/behat/classes/util.php
+++ b/lib/behat/classes/util.php
@@ -32,6 +32,7 @@ require_once(__DIR__ . '/behat_config_manager.php');
 
 require_once(__DIR__ . '/../../filelib.php');
 require_once(__DIR__ . '/../../clilib.php');
+require_once(__DIR__ . '/../../csslib.php');
 
 use Behat\Mink\Session;
 use Behat\Mink\Exception\ExpectationException;
@@ -131,6 +132,35 @@ class behat_util extends testing_util {
         self::store_database_state();
     }
 
+    /**
+     * Build theme CSS.
+     */
+    public static function build_themes() {
+        global $CFG;
+        require_once("{$CFG->libdir}/outputlib.php");
+
+        $themenames = array_keys(\core_component::get_plugin_list('theme'));
+
+        // Load the theme configs.
+        $themeconfigs = array_map(function($themename) {
+            return \theme_config::load($themename);
+        }, $themenames);
+
+        // Build the list of themes and cache them in local cache.
+        $themes = theme_build_css_for_themes($themeconfigs, ['ltr'], true);
+
+        $framework = self::get_framework();
+        $storageroot = self::get_dataroot() . "/{$framework}/themedata";
+
+        foreach ($themes as $themename => $themedata) {
+            $dirname = "{$storageroot}/{$themename}";
+            check_dir_exists($dirname);
+            foreach ($themedata as $direction => $css) {
+                file_put_contents("{$dirname}/{$direction}.css", $css);
+            }
+        }
+    }
+
     /**
      * Drops dataroot and remove test database tables
      * @throws coding_exception