Hello,
With the help of the below code, you will stretch input to fullwidth. Enter it to the “Global Custom CSS” area that is in Theme Settings -> Custom CSS.
.single-product .entry-summary div.quantity,
.single-product .entry-summary div.quantity input[type="number"] {
width: 100%;
}
.single-product .entry-summary div.quantity input[type="button"] {
min-width: 50px;
}
I hope this helps and have a brilliant day ahead! If there’s anything else that I can help with you, do let me know.
Kind Regards