Moodle themes can be set at the site level, course level or user level, depending on how the site has been configured. If as a teacher you are unable to change the theme of your course this has been set at the site level.
How to Change the Theme of Your Choice:
Moodle Administrators: To allow teachers to change the theme of a course, go to: Administration Block, Site Administration, Appearance, Themes, Theme Settings, Allow Course Themes
The default setting is not to allow teachers to select a course theme. Enter a tick mark to allow teachers to select their own theme.
To Change the Theme of a Course:
Note: Being allowed to choose the theme of a course id controlled by the Moodle Administrator for a site. If you do not have the option to select a theme when following the directions below, you have not been given this privilege at your Moodle site.
Directions and screenshots are from Moodle 2.2. The location of “Force theme” has changed with newer versions. However, the terminology of “Force theme” has remained the same. In Moodle version 2.5, “Force theme” is located in the Appearance collapsed menu.
1. In the Settings block, under Course Administration, select Edit Settings. Or, in version 2.5, Administration block, Course Administration, Edit settings.
2. On the Edit Settings page, scroll down until you see the drop-down menu choice: Force theme:
3. Click on the Force theme “Do not force” drop-down menu and you will then see a list of theme choices.
4. Select your theme choice and Save changes. You will then see your course displayed in the new theme when you return to the Topics Outline page in your course.
*Locating “Force theme” in Moodle 2.5+
New Themes in Moodle 2.0