The YaleSites Custom offering is available to organizations that require a unique look and feel and/or complex functionality beyond what the YaleSites Templates offer. Creating a custom site requires advanced technical skills and therefore requires a staff member or external vendor with those skills to build the theme.
You can only work on the theme files in your YaleSites development environment (dev). The only files that will be migrated from your local environment to YaleSites dev environment are theme theme files, via interaction with Github Enterprise. All content types, views and content entry must be done in the YaleSites development environment.