Thank you very much.
I made the changes as you suggested and it worked!
And even managed to make other changes.Changing the stylesheets seemed to be very complicated and enormously time consuming looked at from the point of view of someone whose only experience with styelsheets is Adobe GoLive. However, your reply made me realise that little changes are very easy to make, and thus 'customising' ones own moodle - at least in a limited way - is really quite straightforward.
For any one else new to this, note that because of stylesheets being cached on servers, the changes you make may not be immediately apparent. Also, the Firefox extension (Web Developer Toolbar) makes experimentation with stylesheets much less opaque.
Thanks again.