Hello,
I actually have a multisite (network) setup for my theme demos but know this can be a bit tricky at times. However, just so I get a better picture of things, you have set up your network (multisite) on a subdomain?
But you are wondering about the installation of themes and plugins being available for added “sites” to the network?
When installing a theme or plugin, this is done from the network admin dashboard. For themes, while logged in, you would go to
My Sites >> Network Admin >> Themes
When uploading, you would then Network Enable it. So when you add a site to the network, then go into that site’s dashboard, you would see the theme there and can then activate it for that site.
Plugins are the same but also a bit different. You can Network Enable a plugin, but you can also leave it disabled and then go into the added Site’s dashboard and activate the plugin from there. There may be some plugins that need to be network-enabled, but most can be enabled from each added site’s dashboard. Themes are the ones that must be Network Enabled from the start.
Hopefully, I understood correctly, but if not, let me know.
Regards,
Andre