Rambox.css

/* FOR RAMBOX, edit service and paste under "Advanced / Custom code" */
function applycss(css){
	var head = document.getElementsByTagName('head')[0];
	var s = document.createElement('style');
	s.setAttribute('type', 'text/css');
	s.appendChild(document.createTextNode(css));
	head.appendChild(s);
}

applycss(`
	[class^='chat'] [class^='content'] [class^='messages'] > [class^='container']  {
	padding-top: 11px;
	margin-bottom: 11px;
        padding-bottom: 0;
	zoom: 0.9;
}

[class^='chat'] [class^='content'] [class^='messages'] > [class^='container'] [class^=dividerEnabled] {
	margin: 11px 0 -11px !important;
}

[class^='chat'] [class^='content'] [class^='messages'] > [class^='container'] [class^=markup] {
	line-height: 1.15;
}

[class^='chat'] [class^='content'] [class^='messages'] > [class^='container'] > [class^='message']  {
	position: relative;
}

[class^='chat'] [class^='content'] [class^='messages'] > [class^='container'] h2 strong {
	font-weight: 600;
	letter-spacing: 0.2px;
}
[class^='chat'] [class^='content'] [class^='messages'] > [class^='container'] h2 time {
    font-size: 14px
}

[class^='chat'] [class^='content'] [class^='messages'] > [class^='container'] h2 [class^='botTag']  {
	background-color: #191919;
	color: #aaa;	
	opacity: 0.5;
	padding: 2px 4px;
}

[class^='chat'] [class^='content'] [class^='messages'] > [class^='container'] [class^=header] [class^='image'] {
	height: 37px;
	width: 37px;
	position: relative;
	top: 2px;
	border-radius: 3px;
}
`);

Wavebox.css

/* As a basic custom css (for Wavebox and similar) */
[class^='chat'] [class^='content'] [class^='messages'] > [class^='container']  {
	padding-top: 11px;
	margin-bottom: 11px;
        padding-bottom: 0;
	zoom: 0.9;
}

[class^='chat'] [class^='content'] [class^='messages'] > [class^='container'] [class^=dividerEnabled] {
	margin: 11px 0 -11px !important;
}

[class^='chat'] [class^='content'] [class^='messages'] > [class^='container'] [class^=markup] {
	line-height: 1.15;
}

[class^='chat'] [class^='content'] [class^='messages'] > [class^='container'] > [class^='message']  {
	position: relative;
}

[class^='chat'] [class^='content'] [class^='messages'] > [class^='container'] h2 strong {
	font-weight: 600;
	letter-spacing: 0.2px;
}
[class^='chat'] [class^='content'] [class^='messages'] > [class^='container'] h2 time {
    font-size: 14px
}

[class^='chat'] [class^='content'] [class^='messages'] > [class^='container'] h2 [class^='botTag']  {
	background-color: #191919;
	color: #aaa;	
	opacity: 0.5;
	padding: 2px 4px;
}

[class^='chat'] [class^='content'] [class^='messages'] > [class^='container'] [class^=header] [class^='image'] {
	height: 37px;
	width: 37px;
	position: relative;
	top: 2px;
	border-radius: 3px;
}

Made for Dark + Cozy default theme.

Updated, Discord keeps obfuscating their selectors shrug.