If your theme does not include support for custom CSS, just go to Site Administration > Appearance > Additional HTML and add the following in the Within HEAD field:
Be sure to include the <style> and </style> tags.
The Within HEAD section will insert whatever you enter in the <head> </head> section of your pages. If you are using multiple themes on your site, be aware that this setting applies across your whole site, not just to one theme.
Hope this helps.