@jlx89 You can just create that directory. It's customizable now with notfound.html - https://docs.cloudron.io/branding/#custom-pages
@lunarshade Can you give some idea on what you are trying to customize? If these are general UI/UX improvement, we are happy to incorporate them into the main product itself.
If you are a hosting provider, we are building out customization features in the hosting provider edition - https://cloudron.io/documentation/hosting-provider/ (write to us at firstname.lastname@example.org for a trial license)