Numbas has always had support for themes, allowing you to completely change how exams are displayed. The Numbas editor, however, only supported a static list of themes installed by the server administrator, so most users were stuck with the default themes.
I’ve just released an update to the Numbas editor which allows any user to upload a .zip package of their own theme, which they can then use in any exam. You could use this to do something simple such as show your institution’s logo instead of the Numbas one, or add some CSS rules to tweak the layout of the page.
I’ve written some brief instructions on how to create your own themes, along with a couple of simple examples, in the Numbas documentation.