How can I apply CSS code to change my mobile menu theme?

[ilektronix]

Hello, I would like to ask how can i apply this css code to change my mobile menu theme

code link: https://codepen.io/made-on-mars/pen/qqEgXP

my website link: https://yourgadgetscy.myshopify.com/

Thank you in advance

[ilektronix]

anyone?