Custom theme.scss path in fileadmin

# Feature Request

I have tried various ways to integrate my own theme.scss, which is then also compiled. This function no longer seems to be available?

the old way did not work any more:

page.includeCSS.theme = /fileadmin/.../Scss/bootstrap5/theme.scss

Wouldn't the SCSS section in the configuration set be a good place to store your own file path?

Start by tracing how page.includeCSS.theme currently handles a /fileadmin/.../Scss/bootstrap5/theme.scss path and where the SCSS section of the configuration set is defined. Confirm whether custom paths are still supported and identify the existing compilation or configuration tests; done means a user can configure and compile a custom theme.scss from fileadmin.

Written by the indexing model from the issue text.