Text style tricks using CSS
1. Add borders to the username text
This code will add black and bold borders to the usernames in the chat message area.
.message-user-name{
font-weight: bold;
color:rgba(0,150,245,1.00);
text-shadow: -1px 0 black, 0 1px black,
1px 0 black, 0 -1px black;
}

2. Add borders to the message text
This code will add black and bold borders to the text in the chat area.
.message-text{
color:#fff;
text-shadow: -1px 0 black, 0 1px black,
1px 0 black, 0 -1px black;
font-weight: bold;
}

To know how to edit web CSS, click this link.