It works on dark theme (I use only dark).

On dark it's light (white) text on dark green background, and it looks fine (I would prefer black or similar, but this works).

Your edit breaks dark design because you get white/grey background.

I went a bit farther and used the built in variable to set color, as well as setting the font weight down to match the "n hours ago" text, and changing " - " to "@" so that the uesrname matched the standard fediverse string that you can put into search boxes

I'll publish this properly as soon as greasyfork sends me an email to authenticate my new account, but in the meantime here's the source. EDIT: Email has yet to arrive 13 hours later, I doubt it's going to. Anyone interested feel free to publish this somewhere it's easier for people to install.

// ==UserScript==
// @name        kbin social add home-instance name to username (modified)
// @namespace   english
// @description  kbin social add home -instance name to username, modified to match style and fediverse formatting
// @include     http*://*kbin.social*
// @version     1.16
// @run-at document-end
// @require       https://cdn.jsdelivr.net/npm/jquery@3/dist/jquery.min.js
// @license MIT
// @grant       GM_addStyle
// ==/UserScript==

$( document ).ready(function() {
    $( ".user-inline" ).each(function() {
        // get username URL and text, then remove username from URL and paste the instance name after username (not if instance is home-instance of kbin.social

        var homeinstance = $(this).attr('href') ;
        var myname = $(this).text().trim();

        var homeinstance2 =  homeinstance.replace( "/u/@" + myname + "@"  , '');

        if( homeinstance2  !=   "/u/" + myname ){ //show nothing if home-instance kbin
            console.log(homeinstance2 );
            $(this).append( "<span>@" +  homeinstance2 +"</span>" );
        }
    });
}); //end each username a href

var style = document.createElement('style');
style.type = 'text/css';
style.innerHTML =   '#content a.user-inline span{color: var(--kbin-meta-text-color); font-weight: 400}' ;

document.getElementsByTagName('head')[0].appendChild(style);