You'll have to make some changes to OCS' main stylesheet. The stylesheet can be found at styles/common.css; the rules you are looking to change are under Main Layout Elements. The edits you will have to make will differ depending on which version of OCS you are using; if get stuck, let us know which version you are using and we should be able to help.
Also, the best way to actually implement the changes is not to edit styles/common.css directly, but to look to it for reference. Create your own stylesheet with the edits you want, and then upload it. to upload a new stylesheet for a conference, log in as a Conference Manager, and under Website Management click the "Conference Style" section and upload from there.