By default, b2evolution blogs are displayed in the ‘standard’ skin.
Readers can choose a new skin by using the skin switcher integrated in most skins.
You can restrict available skins by deleting some of them from the /blogs/skins folder. You can also change the default skin or force a specific skin. Actually, you should change the default skin and delete the standard skin, as this one has navigation links at the top that are only good for the sake of the demo. These would be a nonsense on production servers! Read the manual on evoSkins!