How to change Yoko Theme Submenu to drop down

Hello all i’m on project to create my own site using wordpress and i’am planing to use yoko theme because it’s beauty and responsive but i have problem to change the sub menu in yoko theme to become dropdwon menu. here a basic css code from yoko theme sub navigation. pls help me to change it to become dropdown menu. i’m edit this theme using child theme. help me all

[CSS code moderated – please post a link to your site]