Starting with the MP6 plugin, and by default in WordPress 3.8, the admin backend has been flattened. This plugin restores classic (3D) borders.
Over the past few years, User-Interface design has moved towards flatness. One only needs look at Windows 8 to see the direction we are headed in. Starting with version 3.8, WordPress joined in that movement, with admin back-end getting a design overhaul. Borders have been removed from the menus, text is enlarged, image icons are replaced with text icons. Much of this is good and a long time coming. Some of it, however, decreases readability and usability.
This plugin gives you several options to tweak the knew look. The main feature (and namesake) is the ability to tweak the borders on the main menu. Additionally, you can set a hover effect for rows (and choose the color). Finally, there are a few misc improvements. The plugin list has blue bars on the left side of all active plugins. These get a 1px break between to visually indicate that are per-plugin, not per-list. Borders on all lists besides the plugin list get consistent bottom-borders (between items). All these options are on the new Admin page under Appearance.