Business author has some restrictions in capabilities versus business analyst and this is one of them. Besides the Light/Dark themes you can not make changes yourself to the theme (or import a custom theme). You can only select Light/Dark or custom theme. With the custom theme option you will be presented with the theme which your colleage with the Analyst license has created for you for this file.
So you should work with the Analyst colleague in your organisation to make sure you have the custom theme (such as a corporate theme) you prefer for this file. He/she would have to save it as such for you.
Note that the custom theme is only available once your colleague with the Analyst license creates it for you (if you start an analysis from scratch as business author you will only have Light/Dark theme options)