Blogger's Template Designer allows you to customize almost any aspect of your blog's appearance through the use of cascading style sheets (CSS). To add custom CSS snippets to your template, go to the Template tab and click Customize. Then, add your code to the field located in the Advanced | Add CSS tab. Changes you make will reflect instantly in a live preview beneath the editor.
Note: Editing CSS allows you to change the look and feel of your blog. For changes to the actual content of your blog, including the adding of gadgets or changes to the layout, click Edit HTML on the Template tab. Also remember that, like other customizations, your CSS will be removed if you change designs.
For a comprehensive list of various CSS tweaks, please see the What can I do with CSS article in our Help Center.