Design

You can apply custom style sheets (CSS) to both block areas and blocks using the Design menu. Editing these styles may cause appearance issues. If you do not see this button then you site administrator may have disabled them. Generally its a good idea to have your theme control CSS for consistency sake, but the Design command can be handy in a pinch if you just need to add some padding to a content block or throw a background color up quickly. 
You can get the Design command at the block area level, or for a specific block by just clicking it in edit mode: 

The Design window gives you several tabs with setting information.

• Fonts controls styles for all text.
• Background lets you pick a color or upload an image for the background area
• Border creates a line around the area you're working in.
• Spacing lets you add a margin or padding to the area you're working in.
• CSS lets you add your own in-line styles if you're comfortable with CSS code. 

The save this style check box lets you save the current settings as a preset. This is really handy if you're going to use the same style somewhere else. After saving a preset it will be available next time you open the Design window on another block or block area.
NOTE: Depending on the Custom Templates and inline code for a specific block, some of these styles may not fully override the block.