Some blocks already contain options to personalize the look or behavior of it, some blocks don't. However, concrete5 has a nice feature which uses CSS to customize the look of a block, even if the block developer didn't add any options to support custom colors, fonts, or anything like that.
You can see the following dialog to edit these, when you click on a block you've already added:
There are several options which apply different CSS rules to the selected block:
