Trello Minimalist - Credit: https://userstyles.org/styles/101491/minimalist-theme-for-trello

trello-minimalist.css

body {
    background: #f8f8f7 !important;
    font-family: "HelveticaNeue-Light", "Helvetica Neue Light", "Helvetica Neue", Helvetica, Arial, "Lucida Grande", sans-serif !important;
    font-weight: 300 !important;
}

h3 {font-weight:300 !important}

.header-woof {display:none !important}

#header {
    background: #F45B5B !important;
    padding: 10px !important;
}

.board-header-btn-name-org-logo {
 -webkit-filter:  contrast(3000%) invert(100%) ; 
}

.js-open-board  {
  background:#979798 !important;
  font-weight:300 !important;;

}

.board-canvas {padding-top:20px}

.js-open-board:hover  {background:#F45B5B !important}
.thumbnail {background:#979798 !important}

.sidebar-section-header-title {text-decoration:none !important;font-weight:300 !important}
#search-text {border-radius:0px !important}

.board-header {margin-bottom:20px !important; padding-bottom: 0px !important}

/* header buttons */
.header-btn.header-notifications, .header-btn.header-boards, 
.header-btn.header-search, .header-btn.header-login, 
.header-btn.header-signup {
    background: none !important;
}

.header-btn:hover {
    background: none !important;
}

.header-user {
    top: 10px !important;
    right: 5px !important;
    font-weight:300 !important
}

.board-header {
    height: 40px !important;
    background: #F45B5B !important;
}

.header-btn-text {font-weight:300 !important}


.board-header-btn-text {
    text-decoration: none !important;
}

.board-header-btn-name  {font-weight:300 !important}

.text {
    text-decoration: none !important;
}

.header-member {
    background: none !important;
}

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

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

.list-area {
    margin-top: 30px !important;
}

.list-header-name {
    color: #F45B5B !important;
  	font-weight: 300 !important;
    text-transform: capitalize 
}

.list {
    background: #fff !important;
}

.list-card:hover {transition: background 0.5s ease-out !important}

.add-list {
  background: none !important;
}
.add-list:hover {
  background: #fff !important;
}

.add-list span {color: #F45B5B !important;}  


.open-card-composer:hover {
    text-decoration: none !important;
    background: none !important;
    color: #F45B5B !important;
}

.window-wrapper {
    background: #fff !important;
}

.window-wrapper h3 {font-weight:normal !important}

.window-wrapper a {text-decoration:none !important}


.window-overlay {
    background: rgba(244, 91, 91, 0.8) !important;
}

strong {font-weight:300 !important}

.js-open-move-from-header {color:#F45B5B !important; font-weight:300 !important}
.window-title-text {font-weight:300 !important}

.button-link {font-weight:300}

.button-link:hover {
    background: #F45B5B !important;
    border-color:  #F45B5B !important;
} 

.checklist-progress-bar-current {background:#F45B5B !important}

.confirm {background:#F45B5B !important}
.ui-state-active {background:#F45B5B !important}