For simple theme changes, or complex ones for that matter, you can easily add !important to corresponding minicart ID's and classes in your own CSS to overwrite the existing CSS.
Use Firebug/Firefox or Inspect Element/Chrome to get the ID and classes you want to overwrite.
Great if you don't want to create an entirely new theme, but just want to tweak the out-of-the-box theme a bit.
For simple theme changes, or complex ones for that matter, you can easily add !important to corresponding minicart ID's and classes in your own CSS to overwrite the existing CSS.
Use Firebug/Firefox or Inspect Element/Chrome to get the ID and classes you want to overwrite.
Great if you don't want to create an entirely new theme, but just want to tweak the out-of-the-box theme a bit.
Suggest adding to: http://minicartjs.com/#customization