Parse front and back matter and extra books according to their stylesheets.

Front and back matter and glossaries may use a stylesheet named frtbak.sty
Extra books may use a stylesheet named usfm_mod.sty

There is a help topic in Paratext with more information, the key information is here:

How do I create a separate stylesheet for front and back matter?

Material entered in front and back matter (or any of the peripheral books in Paratext) may need to use a structure of styles which don't follow the rules of the USFM standard. You may also want to use different formatting for a particular style from that used in the regular Bible text. To allow for such differences, these peripheral books need to use a separate stylesheet.
The stylesheet must be in the project folder in UTF-8 encoding with the name frtbak.sty

Add entries to this file for any markers used in front and back matter which should be handled differently in these books from the same markers in Biblical books. This stylesheet does not function like the custom stylesheet, custom.sty, which allows customisation of the standard stylesheet by adding just the desired changes. The frtbak.sty stylesheet effectively replaces the standard stylesheet for particular books and so all markers used in any of those books have to be defined in their entirety.
The frtbak.sty stylesheet will be used for all Paratext books with \id lines containing FRT, BAK, XXA to XXG, OTH, INT, CNC, GLO, TDX, and NDX.

If an Extra book (XXA to XXG) contains a Bible module, the frtbak.sty stylesheet is not used for that Extra book. The stylesheet for an Extra book containing a Bible module is usfm_mod.sty

Locate the stylesheet parsing entry point and existing handling for custom.sty, then trace how Paratext book IDs are classified. Add support for frtbak.sty for the listed front, back, and peripheral books, and usfm_mod.sty for Extra books containing a Bible module; done means each case uses the specified stylesheet.