How to restrict customization of sites in SharePoint Online?

Hey all,

So I am looking to create sites for end users where I will set company theme and branding. I can restrict users from changing the branding with permissions but I want to extend this to users who have higher permission levels as well. Essentially im looking to restrict themes to just the company ones so even if they try to change it they are only presented with the themes I have specified. Is this possible?

Thanks in advance