Here is how you can add custom CSS to a block.

1. Select the gear icon from the top menu bar. Choose "Design".
2. Select "Customize" for the theme you're using.
3. Scroll down to the bottom, where you'll see a "custom CSS" input box.
4. Here, you could create a class with your styling. Be sure to save.
5. Then... simply apply the class to the block you want. Do this by using the Design option for that block. Choose the gear icon in the menu, and add the class under "Advanced Custom Class".

Hope that helped.

Another option is to create custom template and add styles (view.css) or css selectors. More flexebale.

C5 DOCS:
http://documentation.concrete5.org/developers/working-with-blocks/w...