installing / running version 4.6

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After many years happily ticking along with a old version of silverstripe, I thought it was about time to come up to date.
I have installed composer & used it to install version 4.6 as a dev in xampp, dev/ build, database okay. that was fine but admin and pages are just text, no theme, all the lessons I have read lead you to an installer, there is no installer now? there is now an app folder as well as a public folder, it is all so different, but I cannot find out how I set the templates, themes etc without breaking it & starting again.

Were you using 3.x before? If so - have you seen this guide?

Thanks for the link, I was using 3.1 so really needed a fresh start, I have got 4.4 downloaded and have installed it ( with the installer) but composer refused to update or upgrade it, so back to a fresh start with 4.6, I have taken my old mysite apart & put it into the app folder, but cannot get any theme to run.
I am missing something probably very obvious, there must be a reason you have taken away the installer, but what has replaced it?

With composer doing most of the heavy lifting, the installer script was just left with writing variables into the .env file, so it was a bit redundant.

On a fresh install, it should be a case of running the composer --create-project command, adding the relevant values to a .env file and running a dev/build.

If you have a ‘default’ install, you should see a file called themes.yml in the config directory. That lists the themes which are available to the site. By default, the ‘simple’ theme is included, and you should see it listed in that themes file. If you create a new theme, put its directory name in the themes file, (you can replace the ‘simple’ theme entry if you’re not using it) and run a dev/build?flush. This will let the framework know that the theme is available and should be included in the stack.