LRR is written in Perl on the server-side with the help of the Mojolicious framework, with basic JQuery on the clientside.
As of v.0.5.5, a basic Client API is available for you to write clients that can connect to a LANraragi instance.
Once you've got a running LANraragi instance, you can basically dive right into the files to modify stuff to your needs. As you need raw access to the files, a native OS install is needed!
I recommend a Linux or WSL install, as the morbo development server only works on Linux.
Said development server can be ran with the npm run dev-server command.
The major difference is that this server will automatically reload when you modify any file within LANraragi. Background worker included!
You'll also probably want to enable Debug Mode in the LRR Options, as that will allow you to view debug-tier logs, alongside the raw Mojolicious logs.