Atahualpa 3.7.12
Wordpress 3.5.2

I've run searches here trying to find the answer but the only relevent threads I found were from 2009 (several wp and theme versions ago). I've just installed newest Atahualpa but cannot find where to change the colors of the built-in calendar widget. Heck, I can't even find the appropriate css file(s) to do it that way.

How the heck do i Make the grey background for days of week dark green (seen on left edge and title fonts) the grid dark green, dates in squares dark green?

Use a code inspector to know exactly what CSS you are looking for, then find and change.

You may need to add !important to the CSS such as

#wp-calendar th {
    background: red !important;
}