Custom Security.ss pages for Subsites

[Sitetools]

I can create a custom Security.ss page but the same custom login page works for all Subsites.
How can I specify what theme should work for what security page? The below makes all login pages use the mycustomtheme1 theme.

---
Before:
  - '#login-forms'
---
SilverStripe\LoginForms\EnablerExtension:
  login_themes:
    - '$public'
    - 'mycustomtheme1'
    - 'mycustomtheme2'
    - 'mycustomtheme3'
    - '$default'
---
---
Name: mytheme
---
SilverStripe\View\SSViewer:
  themes:
    - '$public'
    - 'mycustomtheme1'
    - 'mycustomtheme2'
    - 'mycustomtheme3'
    - '$default'

[Sitetools]

Hi,
Do you have any suggestions as to how I get different templates working for different site logins?
I have another client asking the same question.