A sample class implementation of Drupal 8's ThemeNegotiatorInterface from https://jimconte.com/blog/web/dynamic-theme-switching-in-drupal-8

ThemeNegotiator.php

<?php
/**
 * @file
 * Contains \Drupal\jcmodule\Theme\ThemeNegotiator
 */
namespace Drupal\jcmodule\Theme;

use Drupal\Core\Routing\RouteMatchInterface;
use Drupal\Core\Theme\ThemeNegotiatorInterface;

class ThemeNegotiator implements ThemeNegotiatorInterface {

    /**
     * @param RouteMatchInterface $route_match
     * @return bool
     */
    public function applies(RouteMatchInterface $route_match)
    {
        return $this->negotiateRoute($route_match) ? true : false;
    }

    /**
     * @param RouteMatchInterface $route_match
     * @return null|string
     */
    public function determineActiveTheme(RouteMatchInterface $route_match)
    {
        return $this->negotiateRoute($route_match) ?: null;
    }

    /**
     * Function that does all of the work in selecting a theme
     * @param RouteMatchInterface $route_match
     * @return bool|string
     */
    private function negotiateRoute(RouteMatchInterface $route_match)
    {
        $userRolesArray = \Drupal::currentUser()->getRoles();

        if ($route_match->getRouteName() == 'user.login')
        {
            return 'seven';
        }
        elseif ($route_match->getRouteName() == 'some.other.route')
        {
            return 'some_other_theme';
        }
        elseif (in_array("administrator", $userRolesArray))
        {
            return 'seven';
        }

        return false;
    }

}

hi,
For example when I navigate to URL like /node/add that time it return simple custom theme.

how to implement code based on condition?

Code looks clean and good. But be aware that this way is negotiateRoute() called twice, which is not good.