Adds a new tab in the block settings to control the block visibility based on a custom field

Drupal 8 Custom Block Access Plugin

<?php

namespace Drupal\slq_blocks\Plugin\Condition;

use Drupal\Core\Condition\ConditionPluginBase;
use Drupal\Core\Form\FormStateInterface;

/**
 * Provides a 'Site Theme' condition.
 *
 * @Condition(
 *   id = "site_theme",
 *   label = @Translation("Site Theme"),
 *   context_definitions = {
 *     "node" = @ContextDefinition("entity:node", label = @Translation("Node"))
 *   }
 * )
 */
class SiteTheme extends ConditionPluginBase {

  /**
   * {@inheritdoc}
   */
  protected function getSiteThemes() {
    $defaultTheme = ['theme1' => 'Site Theme 1'];
    $themes = \Drupal::service('entity_field.manager')->getFieldStorageDefinitions('node')['field_site_theme']->get('settings')['allowed_values'];
    $themes = array_merge($defaultTheme, $themes);
    return $themes;
  }

  /**
   * {@inheritdoc}
   */
  protected function getCurrentNodeSiteThemeId() {
    $node = $this->getContextValue('node');
    $defaultThemeId = 'theme1';

    if ($node && $node->hasField('field_site_theme') && !$node->field_site_theme->isEmpty()) {
      return $node->field_site_theme->value;
    }
    else {
      return $defaultThemeId;
    }
  }

  /**
   * {@inheritdoc}
   */
  public function buildConfigurationForm(array $form, FormStateInterface $form_state) {
    $siteThemes = $this->getSiteThemes();
    $currentValues = $this->configuration['site_themes'] ?? [];

    $form['site_themes'] = [
      '#type' => 'checkboxes',
      '#title' => $this->t('Select the site themes you want this block to appear on:'),
      '#default_value' => $currentValues,
      '#options' => array_map('\Drupal\Component\Utility\Html::escape', $siteThemes),
      '#description' => $this->t('If you select no values, the condition will evaluate to TRUE for all users.'),
    ];
    return parent::buildConfigurationForm($form, $form_state);
  }

  /**
   * {@inheritdoc}
   */
  public function defaultConfiguration() {
    return [
      'site_themes' => [],
    ] + parent::defaultConfiguration();
  }

  /**
   * {@inheritdoc}
   */
  public function submitConfigurationForm(array &$form, FormStateInterface $form_state) {
    $this->configuration['site_themes'] = array_filter($form_state->getValue('site_themes'));
    parent::submitConfigurationForm($form, $form_state);
  }

  /**
   * {@inheritdoc}
   */
  public function summary() {
    $siteThemes = array_intersect_key($this->getSiteThemes(), $this->configuration['site_themes']);
    if (count($siteThemes) > 1) {
      $siteThemes = implode(', ', $siteThemes);
    }
    else {
      $siteThemes = reset($siteThemes);
    }
    if (!empty($this->configuration['negate'])) {
      return $this->t('The site theme is not @siteThemes', ['@siteThemes' => $siteThemes]);
    }
    else {
      return $this->t('The site theme is a member of @siteThemes', ['@siteThemes' => $siteThemes]);
    }
  }

  /**
   * {@inheritdoc}
   */
  public function evaluate() {
    if (empty($this->configuration['site_themes']) && !$this->isNegated()) {
      return TRUE;
    }
    $siteThemeId = $this->getCurrentNodeSiteThemeId();
    return (bool) array_key_exists($siteThemeId, $this->configuration['site_themes']);
  }

  /**
   * {@inheritdoc}
   */
  public function getCacheContexts() {
    return [];
  }

}