Click the pencil icon on the top left

As soon as you have clicked the pencil icon the page will be editable. Two tools will be anabled, the toolbox and the inspector bar. You will use the toolbox to add/edit your content and the inspector bar to add styling to elements.

When you are done with your changes, click the tick to save your changes. Click the green Save icon on the bottom right corner to create the theme. You will be redirected to the preview of your theme. From there, you can download your theme as a zip file and ready to upload on the server.