If you want to update your branding and theme after you've created your membership or during the membership process follow these instructions.
Click on three dots to open secondary dropdown and select 'Edit membership'.
Once you've done that then navigate to the Branding and Theme step.
Branding tab allows you to customize your branding. More information on how to customize your branding can be found here.
Theme tab contains settings for your theme. More on this tab can be found in this article.
Finally once you've done this step then click 'Save' to confirm your changes.