Pretty, Minimal Trello Styles. Credit to http://userstyles.org/styles/80264/trello-theme-minimal-orange for a majority of this code.

gistfile1.css

html, body, input, select, textarea {font-size: 13px !important;} 

#header {background: #2B2B2B !important;}

.header-btn.header-notifications, .header-btn.header-boards, 
.header-btn.header-search, .header-btn.header-login, 
.header-btn.header-signup {
    background: #6B6B6B !important;
    }
.header-btn:hover {background: #868686 !important;}

#header-search input[type="text"] {background: #6B6B6B !important;}
#header-search input[type="text"]:focus {background: #fff !important;}

#board-header .board-name {font-weight: normal !important;}

#board {background: #fff !important;}


.list {
    border: none !important;
    background: transparent !important;
    box-shadow: none !important;
    border-radius: 0 0 0 0 !important;
    }

.list-title {margin-left: 11px !important;}
.list-title h2 {
    color: #E53B0B !important; 
    text-transform: uppercase !important; 
    font-size: 14px !important; 
    line-height: 24px !important;
    font-weight: bold !important;
    letter-spacing: 1px !important;
    }

.list-card {
    border-color: #D6D6D6 !important;
    border: none !important;
    border-bottom: 1px solid #ddd !important;
    border-radius: 0 0 0 0 !important;
    padding: 6px 8px 5px !important;
    }

.list-card.selected, .list-card.active-card {      
    /* background: #FFCEA4 !important;     
    border-color: #FFCEA4 !important;   
    box-shadow: 2px 2px 0px #FC9E4E !important; */
    }
.list-card .card-operation {background-color: transparent !important;}
    
.list-card .badges {
  position: absolute;
	top: 5px;
	right: 20px;
  }

.list-card-title {
	width: 81%;
    }
.list-gradient-top, .list-gradient-bottom {background: none !important;}

.open-card-composer {
    padding: 7px 12px !important;
    margin-left: 3px !important;
    margin-right: 1px !important;
    text-transform: uppercase !important;
    font-size: 10px !important;
    border-radius: 0 !important;
    }
.open-card-composer:hover {
    background: #646464 !important; 
    color: #fff !important;
    text-decoration: none !important;
    }




.card-label {text-shadow: none !important;}


.button-link:hover {
    background: -moz-linear-gradient(center top , #676767 0%, #3F3F3F 100%) repeat scroll 0 0 transparent !important;
    border-color: #3F3F3F !important;
    }
    

.pop-over-list li > a:hover,
.uploader:hover .fakefile {background: #424242 !important; border:none !important;}


input.primary[type="submit"], input.primary[type="button"], 
button.primary, .button.primary {
    background: -moz-linear-gradient(center top , #676767 0%, #3F3F3F 100%) repeat scroll 0 0 transparent !important; }


.attachment-list li .preview .check-icon {background-color: #676767 !important;}
.attachment-list li .preview.is-cover {background-color: #676767 !important;}


.extra-large-window .board-wrapper.disabled-all-widgets .board-side-btn.right-side .arrow, 
.large-window .board-wrapper.disabled-all-widgets .board-side-btn.right-side .arrow, 
.medium-window .board-wrapper.disabled-all-widgets .board-side-btn.right-side .arrow {
    border-right-color: #757575 !important;}  
.extra-large-window .board-side-btn.right-side .arrow, 
.large-window .board-side-btn.right-side .arrow, 
.medium-window .board-side-btn.right-side .arrow {
    border-left-color: #757575 !important;}  

.extra-large-window .board-side-btn:hover, 
.large-window .board-side-btn:hover, 
.medium-window .board-side-btn:hover {background: #ADADAD !important;}