To improve things, click on the gear icon on the top right.
To start, here’s the stock “white on white” Yahoo Mail interface: Maybe some day, or perhaps there’s a third party extension to your Web browser offering that capability, but within the Yahoo world it’s likely just sitting somewhere on a to-do list. Still, there are some things you can tweak and improve with Yahoo Mail to change the user experience, though I’m sorry to report that as of this writing you cannot upload your own custom background image. Yahoo Mail recently released a substantial redesign that gave the popular Web-based email service a lot more customization options, which was much needed as prior it felt like, well, 2005 all over again! The new interface is more elegant and powerful, but even with this redesign, there are only limited customization options, certainly fewer than on Gmail or other Web-based email services.
