White border of the sidebar

You can try achieving this by adding the following CSS code in the Custom CSS option of your theme on the below path.

Admin Area -> Appearance -> Customize -> Additional CSS

#secondary .widget.widget_sow-image {
    padding: 0;
}

#secondary .widget.widget_sow-image img {
    width: 100%;
}

Thank you!
Just one more question: How to change the background color? And can I put a different background color on each Sidebar Widgets?

Just one more question: How to change the background color?

Use below CSS code to change it.

#secondary aside.widget {
    background-color: #f5f5f5;
}

You can change the color value in the above code to whatever you want to use by referring the following pages.

https://www.w3schools.com/html/html_colors.asp
https://www.w3schools.com/html/html_colorvalues.asp
https://www.w3schools.com/tags/ref_colorpicker.asp

And can I put a different background color on each Sidebar Widgets?

You can use following CSS code instead of above to achieve this.

#secondary aside.widget:nth-child(1) {
    background-color: #f5f5f5;
}
#secondary aside.widget:nth-child(2) {
    background-color: #2196F3;
}
#secondary aside.widget:nth-child(3) {
    background-color: #f5f5f5;
}
#secondary aside.widget:nth-child(4) {
    background-color: #f5f5f5;
}
#secondary aside.widget:nth-child(5) {
    background-color: #f5f5f5;
}
#secondary aside.widget:nth-child(6) {
    background-color: #FFEB3B;
}

Thank you!

You are most welcome here ??