my guess is that you aren't importing your stylesheets correctly. just b/c you have a default.css and 3c-fixed.css in your folder doesn't mean your style is using them if you aren't pointing to them in your style.css. CSS files don't work that way (unlike PHP files).
/* Reset browser defaults */
@import url('../thematic/library/styles/reset.css');
/* Apply basic typography styles */
@import url('../thematic/library/styles/typography.css');
/* Apply basic image styles */
@import url('../thematic/library/styles/images.css');
/* Apply basic layout styles */
@import url('../thematic/library/layouts/3c-fixed.css');
/* Apply default styles */
@import url('../thematic/library/layouts/default.css');
/* Prepare theme for plugins */
@import url('../thematic/library/styles/plugins.css');
if you child theme's style.css looks like the above then you are pulling in thematic's style sheets and you can change the default.css in your folder and you'll never see a change in the site until you change the @import (following assumes they are in the same folder as your style.css
/* Apply basic layout styles */
@import url('3c-fixed.css');
/* Apply default styles */
@import url('default.css');