By default, b2evolution blogs are displayed using an evoskin.
You can change the skin used by any blog by editing the blog settings in the admin interface.
You can download additional skins from the skin site. To install them, unzip them in the /blogs/skins directory, then go to General Settings > Skins in the admin interface and click on "Install new".
You can also create your own skins by duplicating, renaming and customizing any existing skin folder from the /blogs/skins directory.
To start customizing a skin, open its "index.main.php" file in an editor and read the comments in there. Note: you can also edit skins in the "Files" tab of the admin interface.
And, of course, read the manual on skins!