Support for skinning/customising the UI

We'd like to add an NHS logo to the UI for our SDE, maybe skin the site to fit our branding, add custom links to documentation relating to our environment, make some local customisations etc. This comes down to two things:

1. Skinning. How can we change the appearance of the site without having to patch our fixes into the UI code whenever we update from upstream?
2. Customisation. What's the current development procedure for enhancing or customising the UI? I've seen the documentation, but it's not clear from that if there's any support for debugging the React code without building a full deployment, which is a) slow, and b) hard to debug because the code is minified. Is there anything to make debugging easier and faster?

Many of our tweaks will be site-specific, not something that should be contributed back to the upstream codebase. It would be great if that could be supported by a clean separation of 'core' and 'custom' UI features.

Start with the UI development documentation linked in the issue and the existing React development procedure. Clarify the supported approach for skinning, site-specific links and customizations, and faster unminified debugging; done should be a cleanly separated customization path that avoids patching upstream UI code.