XBackBone/app/Controllers/ThemeController.php

<?php

namespace App\Controllers;

use Psr\Http\Message\ResponseInterface as Response;
use Psr\Http\Message\ServerRequestInterface as Request;

class ThemeController extends Controller
{
    /**
     * @param Response $response
     *
     * @return Response
     */
    public function getThemes(Response $response): Response
    {
        $apiJson = json_decode(file_get_contents('https://bootswatch.com/api/4.json'));

        $out = [];

        $out['Default - Bootstrap 4 default theme'] = 'https://bootswatch.com/_vendor/bootstrap/dist/css/bootstrap.min.css';
        foreach ($apiJson->themes as $theme) {
            $out["{$theme->name} - {$theme->description}"] = $theme->cssMin;
        }

        return json($response, $out);
    }

    /**
     * @param Request  $request
     * @param Response $response
     *
     * @return Response
     */
    public function applyTheme(Request $request, Response $response): Response
    {
        if (!is_writable(BASE_DIR.'static/bootstrap/css/bootstrap.min.css')) {
            $this->session->alert(lang('cannot_write_file'), 'danger');

            return redirect($response, route('system'));
        }

        file_put_contents(BASE_DIR.'static/bootstrap/css/bootstrap.min.css', file_get_contents(param($request, 'css')));

        return redirect($response, route('system'));
    }
}