In WordPress Multisite you need to change which themes are available network wide:
1.- Click on Themes > Installed Themes in the "Network Admin dashboard" area.
2.- Select the themes you want to make available to all sites. This can be one two different ways:
- Activating them individually by selecting “Network Enable” under each Theme title you wish to activate.
- Activating more them all at once using the checkboxes and bulk actions drop-down menu.
3.- After selecting your items and bulk action, you must then click the “Apply” button to ensure your selected changes are finalized.
4.- Once enabled, the themes will be made available for use by site admin users and be listed in
Appearance > Themes in the "Site Admin dashboard".
Keep in mind the difference of existing configurations in WordPress Multisite, you have "Network Admin dashboard" and "Site Admin dashboard"
Please let us know if this solves the issue.
Carlos R. Hernández