How to change font back to "Default" again?


Hi: I inadvertently changed the “Font Face” (under “Format”) from “Default” to “Roboto Regular”, but now I can’t find the “Default” font again, so I’m stuck with non-default fonts (none of which seem to be the default font, or as nice to read as the default font). This seems to apply to all books I read now: when I open a new book the Font Face briefly reads “Default” but then within maybe 1/2 second it changes to “Roboto Regular”. I miss the default font and very much want to find it again, but I must be missing something obvious as there seems no way to set “Default” as the “Font Face”. Many thanks for any help!