How to manage your MediaWiki skins?
To review the available skins for your MediaWiki, first login as administrator. Then click on Preferences 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 on Preview next to it. Once you choose the desired skin, select it and click on Save 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.