Custom css code

[Sawadcb]

I have an image banner in the website that ive been trying to fix, i want it to fill the screen on mobile view, i tried adding this code 

in the custom css section on the builder, but it keeps giving me error, and adding it on base.css applies it to all banners which I don't want, any idea what can i do?

[Sawadcb]

https://f56daa-ac.myshopify.com/?_ab=0&_fd=0&_sc=1

[Sawadcb]

okay please help me with the steps of this😅 im not a developer but kinda familiar with codes. so, how do i add a custom class? or the alternative, what is the actual selector for my banner image? 

thankss!!

[Sawadcb]

I can't find the section where i find the css class or class of my element, I click on the banner, but i can't find "advanced" or "style" settings. see screenshots below

   

[Sawadcb]

okay so i found the class and edited the code accordingly, it worked!!! thank you so much !!!!!