Hello,

Please add this code to your custom CSS box:

div.wpcf7-mail-sent-ok {
    border-color: transparent !important;
}

This code is for the green lines after sending a contact.
Add this code:

div.wpcf7-validation-errors {
 border-color: transparent !important;
}

to remove the yellow lines that appears when you send a non valid contact.
Let us know.

Best regards!