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..b24ea485065 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
@@ -397,6 +427,37 @@ class behat_util extends testing_util {
         initialise_cfg();
     }
 
+    /**
+     * Restore theme CSS stored during behat setup.
+     */
+    public static function restore_saved_themes(): void {
+        global $CFG;
+
+        $themerev = theme_get_revision();
+
+        $framework = self::get_framework();
+        $storageroot = self::get_dataroot() . "/{$framework}/themedata";
+        $themenames = array_keys(\core_component::get_plugin_list('theme'));
+        $directions = ['ltr', 'rtl'];
+
+        $themeconfigs = array_map(function($themename) {
+            return \theme_config::load($themename);
+        }, $themenames);
+
+        foreach ($themeconfigs as $themeconfig) {
+            $themename = $themeconfig->name;
+            $themesubrev = theme_get_sub_revision_for_theme($themename);
+
+            $dirname = "{$storageroot}/{$themename}";
+            foreach ($directions as $direction) {
+                $cssfile = "{$dirname}/{$direction}.css";
+                if (file_exists($cssfile)) {
+                    $themeconfig->set_css_content_cache(file_get_contents($cssfile));
+                }
+            }
+        }
+    }
+
     /**
      * Pause execution immediately.
      *
diff --git a/lib/outputlib.php b/lib/outputlib.php
index e7cb41e7c39..fb4a29c98f9 100644
--- a/lib/outputlib.php
+++ b/lib/outputlib.php
@@ -179,12 +179,13 @@ function theme_get_css_filename($themename, $globalrevision, $themerevision, $di
  * @param theme_config[] $themeconfigs An array of theme_config instances.
  * @param array          $directions   Must be a subset of ['rtl', 'ltr'].
  * @param bool           $cache        Should the generated files be stored in local cache.
+ * @return array         The built theme content in a multi-dimensional array of name => direction => content
  */
-function theme_build_css_for_themes($themeconfigs = [], $directions = ['rtl', 'ltr'], $cache = true) {
+function theme_build_css_for_themes($themeconfigs = [], $directions = ['rtl', 'ltr'], $cache = true): array {
     global $CFG;
 
     if (empty($themeconfigs)) {
-        return;
+        return [];
     }
 
     require_once("{$CFG->libdir}/csslib.php");
@@ -212,7 +213,7 @@ function theme_build_css_for_themes($themeconfigs = [], $directions = ['rtl', 'l
                 css_store_css($themeconfig, $filename, $themecss[$direction]);
             }
         }
-        $themescss[] = $themecss;
+        $themescss[$themeconfig->name] = $themecss;
 
         if ($cache) {
             // Only update the theme revision after we've successfully created the
diff --git a/lib/tests/behat/behat_hooks.php b/lib/tests/behat/behat_hooks.php
index c7b53df6b46..115765a97f0 100644
--- a/lib/tests/behat/behat_hooks.php
+++ b/lib/tests/behat/behat_hooks.php
@@ -367,6 +367,16 @@ class behat_hooks extends behat_base {
         behat_util::reset_all_data();
         error_reporting($errorlevel);
 
+        if ($this->running_javascript()) {
+            // Fetch the user agent.
+            // This isused to choose between the SVG/Non-SVG versions of themes.
+            $useragent = $this->getSession()->evaluateScript('return navigator.userAgent;');
+            \core_useragent::instance(true, $useragent);
+
+            // Restore the saved themes.
+            behat_util::restore_saved_themes();
+        }
+
         // Assign valid data to admin user (some generator-related code needs a valid user).
         $user = $DB->get_record('user', array('username' => 'admin'));
         \core\session\manager::set_user($user);