how to adjust position add to cart button on mobile phone

[PeaceDoe]

hello, yesterday I asked question in the community about how to adjust position "add to cart" button on website

i got the answer like this :

add this code at the end of your CSS file (Themes -> Edit code -> Assets -> theme.css )

.grid__item {
position: relative;
min-height: 450px;
}

.grid__item form {
position: absolute;
bottom: 0px;
margin-bottom: 15px;
}

@media only screen and (max-width: 768px) {
.grid__item {
min-height: 350px;
}
}

on my computer website, i see the button is tidy 

but on my mobile phone ,my store change like this

There is a huge gap in the middle of the button

my shopify store theme is dubet

my shopify store url is :

https://8ebr5g11vrsdzpz9-56140005553.shopifypreview.com/

please tell me what should i do to adjust the add to cart button on mobile phone 

thank you very much and have a nice day : )

[PeaceDoe]

sorry,now I let the problem show up, you can visit it on your mobile browser to have a look

my shopify store rul is:

https://8ebr5g11vrsdzpz9-56140005553.shopifypreview.com/

[MS-WEB-DESIGNER]

Dear @PeaceDoe 

We will be happy to help you.