Allow loading custom CSS from file via env

[mrjovanovic]

I've tried to find this in the documentation and previous issues, but haven't been able to.

It could be useful to pass a path to the custom css as an environmental argument. This would take the form of something like CUSTOM_CSS_PATH. CSS would be loaded from a file instead of a text box.

In the settings interface, one would be provided with a selection of using the default file (path from environmental variable), uploading a custom file, or inputting the custom CSS as in a text box.

Moving custom css to a file would allow versioning and easier deployment of containers.