Add border and background color to single post

Hi all

I need help to customize my post’s list
I want to add border and background color to the single post
I tried to use this code written by Tom one years ago
.wp-show-posts-inner {
padding: 20px;
border: 1px solid #000000;
background: #f1f1f1;
}
but it doesn’t work in my web site
I customized the ccs of the plugin is it the correct way to proceed?

Thank you in advance
Sara

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