First you have to differ between two kinds of styles:
- there are style packages available in the customer area at WoltLab.com (BlueSpace, Pure.Chrome, Volcano-X und Sunrise) and the WoltLab Pluginstore
- and there are styles directly exported from the style editor.
Depending on which kind of style you have you'll install them different ways.
Style packages are installed via the package API (ACP -> System -> Packages -> Install new package) - styles exported directly from the style editor are installed via the style system (ACP -> Design -> Styles -> Import new style).
Generally you can differentiate them by the file extension.
Styles from the style editor have the file extension .tgz whereas style packages have the extension .tar - that's the normal case so that differences are possible.
If you like to install a single style from a style package (style family) you can unpack the style package and install the single style you wish via the style system. But remember that there won't be an update hint in the ACP because single styes are no plugins.