You can create a plugin which makes changes to the admin dashboard. All you need to do is create two functions, one to change the top level toolbar, one to change the sidebar. Then you can add some css to style them and that should get you in the right direction.
// If you want to change top level admin/toolbar bar
add_action( 'wp_before_admin_bar_render', 'my_admin_bar_render' );
function my_admin_bar_render() {
global $wp_admin_bar;
// remove all top level toolbar items
foreach ($wp_admin_bar->get_nodes() as $node) {
$wp_admin_bar->remove_node($node->id);
}
$wp_admin_bar->add_menu( array(
'parent' => false,
'id' => 'my-logo',
'title' => '<img src="http://example.com/logo.png" alt="Blavatar" class="blavatar" height="16" width="16">',
'href' => "http://example.com"
));
$wp_admin_bar->add_menu( array(
'parent' => false,
'id' => 'my-toolbar-1',
'title' => 'Some toolbar item',
'href' => "http://example.com"
));
}
// If you want to change the side bar
add_action( 'admin_menu', 'my_admin_menu' );
function my_admin_menu() {
global $menu;
$menu = array(
array(
'My menu one',
'',
'http://example.com',
'',
'my-menu-class',
'',
'div'
)
);
}