Because of the use of Font-Awesome icons and the positioning of these particular buttons, applying the standard .button class makes them look very strange. Really, they are more like a toolbar than form submit buttons. Thus they were given their own class. If you wish to make changes to these buttons, please use the provided Style Property: Wiki Sidebar > Search Button.
I have gone ahead and make some tweaks to this property for the next release so that it uses a lighter palette color for the background and a background gradient by default. The property also allows you to change the button icon color starting in the same release.
I have also tweaked the CSS somewhat in vBulletin 4 so that background gradients are applied.
vBulletin 3's button styles are inherited from the user agent, so there is no style basis to use. Therefore, in vBulletin 3, the current search button style has been unchanged.