How to include a custom.css

edit 2:

I have the custom css included. It is possible to open the file via the browser, but the css file has no effect (has been used for other websites and includes only design changes for "h2" "h3" (size and color) .

The Theme which is used is [Bootstrap4].

edit 1: Sorry, the button "add" was not visible for me (Safari), with Firefox it works ;-)

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Sorry for asking, but I can't find a way forward.

I've tried these examples - independent:

Var 1: /grav/user/themes/custom/custom.css
var 2:

Unfortunately, the file will not be loaded.

Thats the href for the bootstrapper css:

Maybe you can provide an example, how to integrate within the field the path to a custom.css

Thanks in advance.