To review the available skins for your MediaWiki, first login as administrator. Then click on the Preferences button in the top right corner.
The next step is to choose the Appearance tab.
In the first section you would see the available skins for your application. The default skin is called Vector. You can preview each of the other ones by clicking the Preview button next to it. Once you choose the desired skin, select it and click on the Save button at the bottom of the page.
You can also change the logo of your wiki by altering the $wgLogo variable in LocalSettings.php to the name of the image you want to use as a logo.