You can install themes on a multisite installation the same way you would with a regular WordPress install. Just upload the theme folder to your wp-content/themes folder to install the theme. Installing a theme is only part of the process for individual blogs to use the themes; you'll need to activate them for the entire blog network or for specific blogs.
To activate a theme for an entire network, click on Themes and then click on Installed Themes in the Network Admin dashboard. Check the themes that you want to enable, select Network Enable in the Bulk Actions drop-down menu, and then click on the Apply button. That's all there is to activating a theme (or multiple themes) for an entire multisite network. The individual blog owners can apply the theme just as you would in a regular, nonmultisite WordPress installation.
To activate a theme for just one specific blog and not the entire network, locate the target blog using the Sites menu option in the Network Admin dashboard...