Modern page Custom Theme by code

Hi

How do I set a custom color theme to a modern page by code? Preferable c#.

I have 2 different site collections and will provision pages under both of them but with different color themes.

I'm able to do it under the root (ctx = http://tenant.sharepoint.com) 

using: 

ctx.Web.ApplyTheme("/_catalogs/theme/15/Palette023.spcolor", null, null, true);

But when I try to do it under a sitecollection (ctx = http://tenant.sharepoint.com/sites/external) I get "File not found".

When I enter the path "http://tenant.sharepoint.com/sites/external/_catalogs/theme/15/Palette023.spcolor" in the browser I find the file so the file is there.

What am I doing wrong? Any other ideas on how to achive this(by code)?

Is it possible to set theme for each site created under the site collection individually?