Thanks for the report. This is a bug; it should of course be inserting the necessary CSS to align custom separators in an expected manner for you.
In the meanwhile though, you do not have to hack this into the output every time you compile:
-
Double-click the Ebook compile format in the left sidebar to edit it.
-
In the CSS compile format pane, paste the following:
.separator { text-align: center; text-indent: 0em; margin-left: 0em; margin-right: 0em; }
This approach will keep the separator from being slightly offset by any indent settings that might be impacting the line otherwise.