Inherit from base theme

I am not 100% sure how your theme is setup, but I assume that you have a line in your .info file telling Backdrop that you have a file called skin.css. Something like this:

stylesheets[all][] = css/skin.css

If you have specified in your .info file that you have a file called skin.css, Backdrop will use that instead of the skin.css that you are inheriting from Basis. If you then delete the file, it will still try loading your skin.css even though it no longer exists because the .info file says that it should exist.

I'm using several subthemes of thesis (which is a contrib version of basis) without any problems with css. I don't need to redeclare skin.css. That's inherited from the base theme.

I think there may be a problem with the way you have declared your subtheme. Can you post the info file and file structure here? Also, be sure basis is in its original location.

For example, this is a basis subtheme that is working fine (no problems with css).

Structure:

Important: template.php must be included, even though it's empty (it contains <?php).

And this is the info file:

type = theme
name = Testing theme
description = A sub-theme of Basis, the default front-end theme for Backdrop CMS.
backdrop = 1.x
base theme = basis
screenshot = screenshot.png

This whole subtheme is inside a folder called testing_theme, which lives inside ROOT/themes