Functions and templates for the user interface that themes can override. Drupal's theme system allows a theme to have nearly complete control
hook_themes_uninstalled(array $themes) Respond to themes being uninstalled. Parameters
hook_template_preprocess_default_variables_alter(&$variables) Alter the default, hook-independent variables
hook_css_alter(&$css, \Drupal\Core\Asset\AttachedAssetsInterface $assets) Alter CSS files before they are
hook_theme($existing, $type, $theme, $path) Register a module or theme's theme implementations.
hook_themes_installed($theme_list) Respond to themes being installed. Parameters
hook_library_info_alter(&$libraries, $extension) Alter libraries provided by an extension.
hook_page_top(array &$page_top) Add a renderable array to the top of the page.
hook_js_settings_build(array &$settings, \Drupal\Core\Asset\AttachedAssetsInterface $assets) Modify the
hook_js_alter(&$javascript, \Drupal\Core\Asset\AttachedAssetsInterface $assets) Perform necessary alterations
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