This is just a simple way to get your input boxes and buttons to look all spiffy and flat like in the example image.
(https://www.simplemachines.org/community/index.php?action=dlattach;topic=27254.0;id=4371)
All using CSS.
Hooray!
1) Go to "Admin>Current Theme's Settings>Modify the style.css file".
2) Find these three values if they exist: input, textarea, button
3) Add this line under all of them: border: 1px solid;
4) Click "Save Changes"
5) Enjoy the tasty goodness.