Gutenberg editor (Block editor)で利用するfont-familyを設定。

functions.phpの設定

add_theme_support( 'editor-styles' );
add_editor_style( 'style-editor.css' );

cssの設定

html[lang="ja"] .wp-block * {
  font-family: 'Open Sans','Hiragino Sans','ヒラギノ角ゴシック','Hiragino Kaku Gothic ProN','ヒラギノ角ゴ ProN W3','Helvetica Neue',Helvetica,Arial,sans-serif !important;
}