Christina, please read this part of the Moodle theme documentation.
You would build a theme "family" with "ell" as your parent theme. (Your users should not be able to select it). Secondly you build your "child" themes. In the "child's" config.php you define "ell" as the parent theme plus an own CSS file in which you define only the body background color. This body background color will overwrite the one in the "parent" theme.
Urs