Removing white blocks on sidebar

Hi there,

if you mean the widget background colors then Customizer > Colors > Sidebar - you can make the color transparent.

To add a line below each post listing you would add this CSS:

.generate-columns .inside-article {
    border-bottom: 1px solid #ccc;
}

The colors are working but the css code doesn't generate any borders though.

the sidebar's still the same after inserting the css code

https://imgur.com/PxUUzl6

Sorry i think i misread the requirement the CSS i provided added a border below each of the posts in the main content.
To add the border below each post in the sidebar you would do this:

.sidebar .wp-block-latest-posts.wp-block-latest-posts__list li {
    border-bottom: 1px solid #ccc;
}