If you have ever used a Wordpress template, you will have a general idea of how design customization works in OJS. There are several themes to choose from that can give the journal a different look.
Find themes by going to Settings > Website > Plugins, then scroll down to Theme Plugins. Check off any that you want to try. They will then be in the Theme dropdown of the Appearance tab. (See screenshot at the bottom of this box).
You can add your own logos, images, and thumbnails in the Setup tab, as well as add text for the footer and sidebar.
Finally, if you have a CSS stylesheet, you can upload it in the Advanced tab to further customize your journal.