[GeoffR]

I have located the string that sets the default size of the gap between columns to 1.5em, but unfortunately the same string is also used to set the default font size for <h2> headers, and maybe other things.

So I don't think it is a good idea to use this patch, but here it is if anyone wants to try it and doesn't mind the side-effects.

Paste into librmsdk.so.1.0.0.yaml:

Spoiler:

ePub multi-column gap size:
  - Enabled: no
  - Description: |
      Sets the default size of the gap between columns.
      BEWARE: Also sets the default size of <h2> headers,
      and maybe other things too.
  - FindBaseAddressString: "\01.5em\0"
  - ReplaceString: {Offset: 1, Find: "1.5em", Replace: "1.3em"}