Customize your SilverStripe Backend, see http://doc.silverstripe.org/framework/en/trunk/howto/extend-cms-interface

silverstripe_flatui_grey_cms.css

.cms .cms-content-fields, .cms .cms-content {
  background: #eee;
}
.cms-menu {
  background: #eee;
  box-shadow: rgba(0, 0, 0, 0.2) 0 0 10px;
}
.cms-menu-list li a {
  background: none;
  border: 0px solid #888;
  text-shadow: none;
}
.cms-menu-list li a:hover, .cms-menu-list li.current a {
  background: rgba(0,0,0,0.2);
  border: 0;
  text-shadow: none;
}

.cms table.ss-gridfield-table tr.title th {
  background: rgba(0,0,0,0.4);
}

.cms table.ss-gridfield-table tr.sortable-header, .cms table.ss-gridfield-table tr td.bottom-all {
  background: rgba(0,0,0,0.4);
}

.cms .ss-ui-button {
  background: rgba(255,255,255,0.6);
  box-shadow: inset none;
  border: 1px solid rgba(0,0,0,0.2);
  text-shadow: none;
}
.cms table.ss-gridfield-table tr th.main {
  background: rgba(0,0,0,0.4);
  text-shadow: 0;
  border-top: 1px solid rgba(255,255,255,0.3);
  border-left: 1px solid rgba(255,255,255,0.3);
  border-bottom: 1px solid rgba(0,0,0,0.8);
}
.cms .ss-ui-button:hover {
  box-shadow: none
}
.cms .ss-ui-button.ss-ui-action-constructive {
  background: #1f9433;
}
.cms-logo-header {
  background: #222;
}
.cms-login-status {
  border: 0;
  border-top: 0;
  padding: 12px 0;
}
.cms-logo {
  display: none
}
.cms-panel .cms-panel-toggle.south, .cms-panel.cms-content-tools .cms-panel-toggle.south {
  box-shadow: none;
  border-top: 1px solid rgba(0,0,0,0.1);
}
.ui-tabs .cms-content-header-tabs .ui-tabs-nav li {
  background: rgba(255,255,255,0.3);
  box-shadow: none;
  border-right: 1px solid rgba(0,0,0,0.1);
}
.cms table.ss-gridfield-table tr.title th {
  border: 0;
}
.cms-content-tools, body.SecurityAdmin {
  background: none;
}
.cms-content-actions, .cms-preview-controls {
  background: rgba(0,0,0,0.05);
  border-top: 0;
  box-shadow: none;
  /*box-shadow: 0px 0px 10px rgba(0,0,0,0.4);*/
}

.field label.left, .jstree-apple a, .cms .ss-ui-action-tabset.action-menus.ss-tabset ul.ui-tabs-nav li a {
  text-shadow: none;
}

.ss-toggle .ui-accordion-header.ui-state-default {
  background: rgba(0,0,0,0.05);
  box-shadow: none;
  border-radius: 1px;
}

.ui-tabs .cms-content-header-tabs .ui-tabs-nav .ui-state-default {
  background: none;
  box-shadow: none;
  border-right: 1px solid rgba(0,0,0,0.2);
}
.ui-tabs .cms-content-header-tabs .ui-tabs-nav .ui-state-active {
  background: rgba(255,255,255,0.5);
}
.field input.text, .field textarea, .field .TreeDropdownField {
  border-radius: 1px;
  border: 1px solid rgba(0,0,0,0.2);
  box-shadow: none;
}
.field input.text:focus, .field textarea:focus, .field .TreeDropdownField:focus {
  border: 1px solid rgba(0,0,0,0.5);
  box-shadow: none;
}
.field {
  border-bottom: none;
  box-shadow: 0 1px 0 rgba(0, 0, 0, 0.1);
}