To use the Theme Designer make sure you have a registered user within SAP cloud platform or register a new one for free.
Export the theme and make sure that in the dialog, you set the checkbox beside Source files + CSS Resources.
You can then upload the theme to your application to give it a specific look corresponding to your corporate design.
To activate the theme, you have to select the Instances.
Then you can switch in the UI Designer between the themes to select the one you need.