You can theme NSS by adding a company/departmental logo to the top left of the NSS tenant homepage which is persisted through all pages (of that tenant) as below in this example for "Hermes".
You will need to prepare a logo with the correct dimensions. In the example above the replacement Hermes logo is 400 x 63.
To apply the logo, log in with admin access and access the admin cog at the top right of the homepage.
Then Organization->Tenant to view Tenant Maintenance'
Select the tenant you want to apply the logo for to view 'Edit Tenant'
Now select the 'Theme' tab and select the 'Header logo image' to upload your new logo. If you wish to check the logo whilst logged in as an admin select 'View Tenant Theme' button on the top right of the screen.
That's it, you can apply different logo's to each tenant.