Hi Sabbir,
That space in your screenshot is space for CSS classes. You shouldn’t place CSS code there.
For it to work, you can place a CSS class of your preference there and add your CSS code targeting the class in Appearance > Customize > Additional CSS.
For instance, I added remove-bottom-margin
here: https://share.getcloudapp.com/2NuzKb1p
And then added this code in Additional CSS:
.remove-bottom-margin {
margin-bottom: 0 !important;
}
Here is an article with regards to adding CSS: https://docs.generatepress.com/article/adding-css/#additional-css
As mentioned, adding it through additional CSS should work.
Hope this clarifies. Feel free to reach out if you’ll need further assistance with regards to this. 🙂