Trello Ultimate Flat Userstyle - Credit: https://userstyles.org/styles/103373/trello-ultimate-flat

trello-ultimate_flat.css

html,body,input,select,textarea,button {
	font:13px 'Tahoma',sans-serif!important;
	line-height:17px!important;
	font-weight:300!important;
}

.list-card a {
	color:#444!important;
}

.list-title .icon-menu {
	right:0!important;
	top:8px!important;
}

.list-gradient-top,.list-gradient-bottom {
	display:none!important;
}

.list-area-wrapper {
	padding-left:10px;
}

.list-area {
	width:3500px!important;
}

.list-card-details {
	padding:3px 4px 2px;
	position:relative;
	z-index:10;
}

.list-header {
	box-shadow:0 rgba(0,0,0,0.2)!important;
	padding:8px!important;
	background:#fff!important;
	color:#3598DB!important;
	z-index:10!important;
	margin:0!important;
}

.list-header h2 {
	font-size:15px!important;
	font-weight:400;
	text-transform:uppercase;
}

.list-header-menu-icon {
	top:5px!important;
	right:0!important;
}

.list {
	background:rgba(252,252,252,.9)!important;
	width:300px!important;
	margin:0 5px!important;
	padding:0!important;
}

.list-cards {
	padding:0px !important;
}

.list.add-list .placeholder {
	display:none;
	padding:7px;
	color:#E84C3D!important;
}

.list-card {
	padding:5px 5px 2px!important;
	margin:4px 0!important;
	border-radius:0;
}

.board-header-btn {
	color:#fff!important;
	font-weight:400!important;
}

/* Change the Board-Canvas Background to a different color or use url to get image */
.board-canvas {
	background: #2D3E50!important;
	background-size:cover!important;
}

#board {
	padding:8px 0 0!important;
}

#header {
	background:#2d3e50!important;
	padding:4px 0 3px!important;
}

.header-user {
	top:10px!important;
	right:5px!important;
}

.board-header {
	height:30px!important;
	background:#E84C3D!important;
	padding:3px;
}

.open-card-composer {
	margin:0!important;
	padding:7px 10px;
}

h2 {
	font-size:18px!important;
	line-height:1.2em!important;
	margin:0 0 9px!important;
	font-weight:400!important;
}


.member {
	border-radius:20px!important;
	border:none!important;
}

.member img {
	border-radius:0!important;
	border:none!important;
}


.fancy-scrollbar::-webkit-scrollbar-track-piece {
	background:none!important;
}

.fancy-scrollbar::-webkit-scrollbar-thumb:vertical,.fancy-scrollbar::-webkit-scrollbar-thumb:horizontal {
	background:#555!important;
}

.open-card-composer:hover {
	background:#ECF0F3!important;
	color:#666!important;
	text-decoration:none!important;
	margin:0;
}

.window-wrapper {
	background:#FFF!important;
	padding:10px 20px!important;
	border-radius:5px!important;
}

.board-widgets {
	background:rgba(255,255,255,0.98)!important;
}

.sidebar-hide-btn {
	top:5px!important;
}

.atMention {
	background:#A3359F!important;
	border-radius:5px!important;
	color:#fff!important;
	font-size:12px!important;
}

code {
	font-family:Menlo,Monaco,"Courier New",monospace!important;
	background-color:#F7F7F9!important;
	border:1px solid #E1E1E8!important;
	border-radius:3px!important;
	padding:3px 4px!important;
	font-size:12px!important;
	color:#D14!important;
}