WorkflowGen comes standard with the Default skin, which can be found in the \wfgen\App_Themes\Default folder.
You can modify the User Portal default CSS file according to your needs by editing the \wfgen\App_Themes\Default\portal\css\Default.css file.
Important: Make sure you save your modifications before you upgrade WorkflowGen, and check the file differences afterward (new styles and images). Alternately, you can create a new skin (e.g. \wfgen\App_Themes\myskin) based on the default skin (\wfgen\App_Themes\Default\*.*) and select this skin on the General tab in the Administration Module Configuration Panel.