After you modify the Theme, you need

Make Aggregated Style cleared

When working with style sheets, make sure CSS aggregation is disabled. It is located in "Administer > Site configuration > Performance". When it is enabled, any alterations will not be reflected on your site until the aggregated styles are cleared. You can enable it again when you're done.

Clear the cache

The .info file is cached. Adding or removing any styles will not be noticed until it is cleared. (Do not confuse this with the theme registry.) To have it cleared, do one of the following:

1. Clear button located at "Administer > Site configuration > Performance".
2. With devel block enabled (comes with devel module), click the "Empty cache" link.
3. Simply visit the theme select page at "Administer > Site building > Themes".

Excerpted from Style Sheet at Drupal web site.