From b2evolution manual

Skins, also called evoSkins, are the front-end of your blog, the part that is visible to the public.

Skins are the preferred way of giving a specific look and feel to your blog but they are not the only way. Please see the chapter about Customization for more details.

[edit] Working with skins

• Working with skins — a quick introduction.
• Skins 2.0 (a.k.a. Themes, Templates...) — the easiest way to completely change the look and feel of a blog.
• Installing skins
• Upgrade skins to a new version
• Edit your skin's CSS file(s)
• Template tags
• Converting WordPress Themes