You can apply a new theme to your at any point via the Course Theme settings. Here's how;

1. Open your course and select the Course Settings tab

2. Select the Theme tab

3. Under the Selected themes title you will see a list of themes available in your account

4. To add a new theme to your course left mouse click on the Theme icon. You will be asked to confirm the selection

Please Note:  In order to commit the new theme to your course you must refresh your web browser after selecting Confirm new theme selection