WordPress and WordPress Multisite comes with two default theme (Twenty Twelve and Twenty Eleven) installed. To extend available themes you’ll need to install extra themes on your install.
Not all themes that work on a single install of WordPress work well with WordPress Multisite.
When choosing a good WordPress Multisite theme look for features such as:
1. No code edits
3. Simplicity of use
All WPMU DEV themes have been completely tested and customized to work with both WordPress and WordPress Multisite.
This means you’re not going to have to upload a single file, edit a single line of html, php or CSS or concern yourself with bugs or issues; they’re ready to use as is!
Any themes downloaded from sites other than WPMU DEV themes should be thoroughly tested by uploading each theme to your WordPress Multisite install, and enabling it just for a test account.
Activate the theme on the test site taking notes of any errors which occur. Only enable the theme network wide once you have fixed them all issues with the theme.