A user stylesheet to make Basecamp a little bit prettier. Custom colors, a bit of jiggery-pokery with the header spacing, hide the milestones calendar from the dashboard page, hide the company logo (if you have one).

basecamp-custom.css

/* Basic color replacements */

a:link, a:visited {
    color: #039 !important;
}

a:hover {
    color: #fff !important;
    background-color: #039 !important;
}

#Header {
    background-color: #65a64c !important;
}

#Header h1 {
    color: #fff !important;
}

#P2P_selector {
    border: 1px solid #485C8A !important;
    color: #000 !important;
}

#Header h3, #Header h3 a:link, #Header h3 a:visited {
    color: #fff !important;
}

#Header h3 a:hover {
    color: #fff !important;
    background: none !important;
}

#Header h3 a.current:link, #Header h3 a.current:visited {
    color: #fff !important;
}

#Header h3 a.current:hover {
    color: #fff !important;
    background: none !important;
}

#Header h1 a:link, #Header h1 a:visited {
    color: #000 !important;
    text-decoration: none !important;
}

#Header h1 a:hover {
    color: #000 !important;
    background-color: #C5E2E9 !important;
    text-decoration: underline !important;
}

#Header a.image:hover { background-color: transparent !important; }

#Header h1 span {
    color: #b4cdaa !important;
}

#Tabs a:link, #Tabs a:visited {
    background-color: #28292c !important;
    color: #FFF !important;
    border: 1px solid #65a64c !important;
    border-bottom: 1px solid #6e6e6e !important;
}

#Tabs ul#MainTabs a:link.current, #Tabs ul#MainTabs a:visited.current,
div#CategoryList a.current:link, div#CategoryList a.current:visited {
    color: #000 !important;
    border-bottom-color: #e8e8e8 !important;
}

#Tabs ul#MainTabs a { margin-left: 2px !important; }

#Tabs a:hover {
    text-decoration: underline !important;
    border-bottom: 1px solid #6e6e6e !important;
}

#Tabs a:active { text-decoration: none !important; }

#Tabs li#AdminTab a:link, #Tabs li#AdminTab a:visited {
    color: #485C8A !important;
    background-color: #C5E2E9 !important;
    border-bottom: 1px solid #6e6e6e !important;
    text-decoration: underline !important;
}

#Tabs li#AdminTab a:hover {
    color: #000 !important;
}

#Tabs li#AdminTab a.current {
    color: #000 !important;
    text-decoration: none !important;
}

span.switch div.menu a.hover,
span.switch div.menu a.hover span {
    background: #f3f4f5 !important;
}

span.switch div.menu a:active,
span.switch div.menu a:active span {
    background: #ffa !important;
}

table.forum_view td.message a:hover {
    color: #039 !important;
}

table.forum_view td.message a span.title:hover {
    color: #fff !important;
    background: #039 !important;
}

/* Hide the logo box and milestones calendar in the dashboard */
div#LogoBox { display: none !important; }
#dashboard_calendar { display: none !important; }

/* Repair the open bar styles */
div#open_bar {
    background: #28292c !important;
    position: relative !important;
    text-align: left !important;
    font-size: 10px !important;
    z-index: 100 !important;
}

div#open_bar div.message {
    padding: 5px 30px !important;
    color: #999 !important;
    margin: 0 !important;
}

div#open_bar ul {
    margin: 0 !important;
    padding: 0 !important;
}

div#open_bar li {
    list-style-type: none !important;
    line-height: normal !important;
    padding: 0 !important;
}

div#open_bar ul.menus {
    padding: 0 25px !important;
}

div#open_bar li.menu {
    float: left !important;
    position: relative !important;
    margin-right: 4px !important;
}

div#open_bar li.menu a {
    position: relative !important;
    display: block !important;
    padding: 4px 5px !important;
    font-size: 10px !important;
    color: #999 !important;
    text-decoration: none !important;
}

div#open_bar li.menu a:hover {
    background: #28292c !important;
}

div#open_bar li.menu a.on {
    color: #ddd !important;
}

div#open_bar li.menu a.on:hover {
    color: #ddd !important;
}

div#open_bar li.menu a.account:hover {
    color: #fff !important;
}

div#open_bar li.menu a.current_account:hover {
    color: #999 !important;
}

div#open_bar li.menu div.items {
    display: none !important;
    position: absolute !important;
    z-index: 5 !important;
    left: 0 !important;
    width: 200px !important;
    background: url(/images/open_bar_bg.png) repeat !important;
}

div#open_bar li.hover div.items {
    display: block !important;
}

div#open_bar li.menu ul.items {
    padding: 0 3px 3px 0 !important;
}

div#open_bar li.item {
    background: #28292c !important;
    border-top: 1px solid #222 !important;
}

div#open_bar li.item a {
    padding-left: 10px !important;
}

div#open_bar ul.items li.last {
    padding-bottom: 5px !important;
}

/* Pills */

html body.todos div.list a.attached_milestone:hover {
  color: #fff !important;
  background: #666 !important;
}

html body.todos div.list a.pill_todo_item.menu_target:hover {
  background-color: #CCC !important;
}

html body.todos div.list a.pill_todo_item.menu_target:hover span.content {
  color: #333 !important;
}

body.todos div.list .active_menu a.pill_todo_item,
body.todos div.list .active_menu a.pill_todo_item.menu_target:hover {
  background-color: #000 !important;
}

body.todos div.list .active_menu a.pill_todo_item.menu_target.busy {
  background-color: #eee !important;;
}

body.todos div.list .active_menu a.pill_todo_item.menu_target span.content {
  color: #fff !important;
}

body.todos div.list .active_menu a.pill_todo_item.menu_target:hover span.content {
  color: #fff !important;
}