About Themes / Skins
Themes, also known as skins, forum designs, or styles improve the style and design of your board. They stylize the layout of your forum (it can be front-end design and back-end design). Themes use stylesheets (CSS files) and HTML-based templates.
Each board comes with a default MyBB theme. It is recommended to not delete or rewrite this theme.
Finding Themes
Themes can be found at the MyBB Extend site (Extend) - MyBB Mods - Themes.
Themes discussion, relases, requests and support can be found at MyBB Community Forums - Themes.
You can also find MyBB themes at lot of other sites on the internet, but be sure that that themes are 1.8.x version compatible. For 1.6 Themes check MyBB 1.6 Documentation, or directly MyBB 1.6 Themes.
Installing themes
Firstly, after you download (usually a ZIP file), extract it somewhere and find in the resulting folder a file named <theme name>.xml or similar… This is the file you will need for the installation (through the Admin CP).
Many authors provide a README file within the resulting extracted folder, which will help you install it correctly. Sometimes there is more complexity than a basic installation as described here.
Uploading the XML File
An XML file is bundled with a theme download. This XML file contains all the templates the theme uses, as well as some other default settings and details about the theme.
The XML file should be uploaded through the Admin Control Panel. So go to Admin Control Panel -> Templates & Style -> Import a theme.
Under Import from choose Local file -> Search and upload your .xml file (file named <theme name>.xml).
You may need to check the Ignore version compatiblity option so you can upload it. And click Import theme to finish.
A tutorial with images can be found here: Theme installation.
Uploading Images
In the folder, you will probably see either a folder of images named similar to the theme, or an images folder with a subfolder (usually named the same as the theme, or similar) of images. Upload the entire folder that contains the images to images/<theme-images-folder-name>, unless the documentation provided with your theme requests otherwise.