how to apply CSS for a Custom Block/View
It depends on what theme you're using. The core Drupal themes are stored under /themes/. If you have a custom theme, it should probably be installed in /sites/all/themes/. Different themes may name their main CSS file differently, but look for .css files within your theme's directory or a subdirectory beneath it.

From there, you'll need to know what ID or class name is associated with the particular block.  You can right click in somewhere in the block and choose "Inspect Element" to examine the source file. Or for even more power, I'd recommend installing the Firebug extension for Firefox. You can find out more about Firebug in this screencast.

For more complicated theming, I'd recommend also installing the Theme Developer module. It will allow you to see what theme file or function is currently outputting a specific area of the page, as well as what files or functions can be used to override it. See the screencast for more detailed information.