just a couple of pointers about changing styles in the default theme.
I see you are using the twenty eleven default theme, when WordPress release an update it is likely the theme will also be updated, this will mean the folder gets deleted and over written.
WordPress 3.4 is in beta stage, with a possible May release, and there will be a few theme changes you may want, if you update you have lost the changes.
Creating Child Themes
Changes you have done will be lost if you upgrade the theme, so to get around this WordPress has a feature called Child Themes, these allow us to make changes without editing the core files, we just create and upload one which is worth doing to protect our changes.
The second is stylesheets are cascading so rather than editing existing styles, and forgetting just where we changed that style, a good practice is just to add our changes at the end of a stylesheet, easy to find if we need to revert back.
In this instance in the child theme we would add in the child themes style.css: