I have a sign up form that I put in sidebar and it is displaying white background (the field for typing e mail address for signup), though my site is black.
Is there a way to make it black?

I made all changes possible in styling forms, sidebars and widgets in ATO.

(Atahualpa 3.6.1
WP latest)

You have to determine the selectors for the form and style them. I use Firebug with Firefox for that.

I added CSS styles and resolved my problem.
I thought that Atahualpa styles would automatically apply to any form I use.

Sometimes they will. It just depends on the plugin.