Fix update stylesheet see #1998

main/inc/lib/display.lib.php

@@ -87,6 +87,7 @@ class Display
 
         if (!empty(self::$preview_style)) {
             self::$global_template->preview_theme = self::$preview_style;
+            self::$global_template->set_system_parameters();
             self::$global_template->setCssFiles();
             self::$global_template->set_js_files();
             self::$global_template->setCssCustomFiles();

main/inc/lib/template.lib.php

@@ -526,9 +526,13 @@ class Template
     /**
      * Set system parameters
      */
-    private function set_system_parameters()
+    public function set_system_parameters()
     {
         $this->theme = api_get_visual_theme();
+        if (!empty($this->preview_theme)) {
+            $this->theme = $this->preview_theme;
+        }
+
         $this->themeDir = self::getThemeDir($this->theme);
 
         // Setting app paths/URLs
@@ -573,11 +577,6 @@ class Template
     {
         global $disable_js_and_css_files;
         $css = array();
-        $this->theme = api_get_visual_theme();
-
-        if (!empty($this->preview_theme)) {
-            $this->theme = $this->preview_theme;
-        }
 
         // Default CSS Bootstrap
         $bowerCSSFiles = [