Change horrible Username banner CSS in UIX Classic Dark [NEED HELP]

[grindelcode]

Im using the UIX Classic Dark Theme. My problem is that those black corners look horrible in the mobile version.
I would like to hide those on the mobile version, if that's not possible that easy I would like to hide them at all.

Here is the CSS:

    .message-userBanner.userBanner {
        margin-left: -14px;
        margin-right: -14px;
        position: relative;  
        margin-top: 5px;
        border-top-left-radius: 0;
        border-top-right-radius: 0;
       
        .userBanner-before {
            left: -1px;
            border-top-left-radius: 3px;          
        }
       
        .userBanner-after {
            right: -1px;
            border-top-right-radius: 3px;          
        }
       
        .userBanner-before, .userBanner-after {
            content: '';
            width: 5px;
            height: 4px;
            background: rgba(0,0,0,.7);
            position: absolute;
            top: -4px;          
        }
    }
}

Thanks in advance!