Again thank you for your reply Mkhalid.
I hace copied the style.css file from Sigma theme./*
Theme Name: Sigma
Theme URI: https://wp-event-organiser.com/extensions/sigma-theme/
Author: Stephen Harris
Author URI: https://stephenharris.info
Description: Sigma Theme is a lightweight and responsive theme, with a fluid grid system that adapts your website to mobile devices and the desktop or any
other viewing environment. The theme is early in development, and feedback is welcomed. The theme features full and with-sidebar page templates, sidebar,
footer and home page widget areas. It is designed to work well with Event Organiser, and should also be compatible with most slider plug-ins and WordPress
SEO. The theme is translation-ready and cross-browser compatible.
Version: 0.1.2
License: GPL v3+
License URI: https://www.gnu.org/licenses/gpl-3.0.html
Tags: responsive-layout, fluid-layout, translation-ready, light, custom-background, custom-header, custom-menu, right-sidebar, full-width-template, light, sticky-post, threaded-comments, white
Text Domain: sigma
*/
/*
Sigma WordPress Theme, Copyright (C) 2013 Stephen Harris
This program is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
Where should I insert the css code.
Thanks
Keith