[xmonad] Workspace selector font size

[Jacek Generowicz]

>> You may also have a `myXPConfig` or something, in which you can add
>> the `font = …` override as well.

Where can I find documentation describing what else can be overridden
here?

`def` isn't the most searchable name, and it seems to apply to many
different situations. The docs seem suggest

> To see what fields can be customized beyond the ones in the example above, the
> definition of the XConfig data structure can be found in XMonad.Core.

that it's here:

  https://xmonad.github.io/xmonad-docs/xmonad-0.17.0.9/XMonad-Core.html#v:XConfig

where there is no mention of `font`.