I'd like the sidebar to be 25% of the width of the Layout, so that it's responsive (scales for viewing on devices.) However the current pixel-only setting is keeping this from happening. Is there a fix?

You could try the following if the class of the sidebar you are referring to is colthree

.colthree {
    width: 25% !important;
}

This isn't really responsive as a 25% sidebar on a 320px screen will only be 80px wide which may not be very readable.

Thanks, I tried that...the problem is that it pushes the widget to the right past the edge of the layout both on the large and mobile screens. When placed inside a media query it doesn't work at all.

Is there a way to force the right sidebar to stack on top of or under the content to its left when viewed on mobile screens? I assume this would be done in a media query but beyond that I'm clueless.

I should add that the fix here (http://forum.bytesforall.com/showthread.php?t=20759) works on images, I just don't know how to get it to work on images within my sidebar widget....

colthree img {width: 100%;}

didn't work.

Because Atahualpa is built with tables I don't think you can do what you want.

Isn't the theme supposed to be responsive, though? I can't be the only person who has come across this problem.

Isn't the theme supposed to be responsive, though? I can't be the only person who has come across this problem.

No, Atahualpa was built before responsiveness became an item.