How to override Gutenberg admin CSS

To override Gutenberg’s styles you need to add your own stylesheet. So hook into enqueue_block_editor_assets and then add your own stylesheet in which you target the selectors you wanna override. In the following example I placed a stylesheet in a custom theme’s assets/ folder.

functions.php:

// Add backend styles for Gutenberg.
add_action('enqueue_block_editor_assets', 'gutenberg_editor_assets');

function gutenberg_editor_assets() {
  // Load the theme styles within Gutenberg.
  wp_enqueue_style('my-gutenberg-editor-styles', get_theme_file_uri('/assets/gutenberg-editor-styles.css'), FALSE);
}

assets/gutenberg-editor-styles.css:

.editor-post-title__input {
  border: 1px solid black;
}

Source: Creating theme editor styles for Gutenberg