Why am I getting a 403 error with this route? I'd like to apply the admin (or other) theme to user /user/login page. Since ThemeKey isn't available I'm attempting to implement via routes. "View admin theme" permission is granted to anonymous.

Using this route:

my_module_name.user_login:
  path: '/user/login'
  options:
    _custom_theme: 'my_custom_theme'


gives the correct theme, but a 403 response on the /user/login page. Why am I getting a 403 response (with the anticipated theme) instead of the standard login page?
Because you don't have any access checks.

If anyone is allowed to access it, you must specify so explicitly with _access: 'true' in an access section.