Add lane Break between billing_first_name and billing_last_name

Hello,

I'm using the Sydney theme and would like to add a line break between the "billing first name" and "billing_last_name" fields in the checkout page.
In other words: make the billing first name field full-width and the billing last name field also full-width in a separate lane.

Here is an illustration of the current and desired situation:

https://ibb.co/xgQqrSd

PS: the website is in Arabic.
You must add at least one product to see the checkout page.

Thanks

The page I need help with: [log in to see the link]