There are three choices here:
The default (global value) is selected under Options/layout (upper icon)/text properties/Text appearance:
Individual subtitles may be changed using the dropdown list in the format menu.
The different appearance types are only fully supported in "software resize", and are only visible in display mode. In edit mode the box is always black. Appearance changes are disabled in hardware resize mode, and will probably not be implemented simply because of lack support for transparent box/solid text functionality in directshow
Boxing values may be saved in .usf and .pac PU2020 format.
Now also added to Swift .xif format both in import and export sections. Note that transparent background box and default value will always be a exported identically (value 7) and since there is no default concept in .xif format then Sub Machine will always take the absolute value (Text with box, Transparent background or Outline) when importing, unless "Import subtitles with default appearance" has been checked.