As of the 4.0.0 version, you may create the following files (relative to the root install path) and they’ll be rendered in the web admin UI:

• Views/Shared/header_custom.cshtml : Render a custom header.
• Views/Shared/footer_custom.cshtml : Render a custom footer. If exists, the standard footer will be supressed.
• wwwroot/css/custom.css : custom css
• wwwroot/js/custom.js : custom javascript

These files are preserved when upgrading to new versions of the software.