Given that multi-user in itself is a deviation from the way things are done, I'm not sure that the final answer will be determined here.
I recommend the following approach, which I think is well-suited for your use:
In your layout:
<html> <head> <%= stylesheet_link_tag "tenant_#{@tenant_name}" %> </head> <body class="tenant-<%= @tenant_name %>"> </body> </html>
In your scss files:
For each tenant (for example, t1) you can:
tenant_t1.css.scss
body.tenant-t1 { ... stylesheets specific to tenant 1 scoped within tenant-specific class ... }
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