We are testing some features of Sharepoint Online, and we want to deploy a new theme, but ONLY on one subsite (test site), not our main site, is that possible?
Or should we make a completly new Sharepoint online main site?
Yes, you can deploy custom themes on a sub-site level.
Add your custom theme at a site collection level where the sub site exists from Site Settings > under
Web Designer Galleries, select
Themes. Click on the folder
15 and upload your Theme here. From here SharePoint takes over and creates a new set of icons based on your colors and make it available from the "Change the Looks" section in addition whit the colors you specified for each element available to change.
Now it should be available from any sub site in the site collection. Head to the subsite of your choice and select Change the look from the Gear icon menu.
Just click on one of the existing looks, and under colors your custom theme should be available.