Update theme to not bold spanning headers

[ddsjoberg]

pharmaverseadam::adsl |> 
  tbl_strata(
    strata = ETHNIC, 
    ~ .x |> 
      tbl_roche_summary(
        by = TRT01A,
        include = SEX
      )
  )

[Melkiades]

So this happens because the default strata header is **{strata}**. The following works:

pharmaverseadam::adsl |> 
  tbl_strata(
    strata = ETHNIC, 
    ~ .x |> 
      tbl_roche_summary(
        by = TRT01A,
        include = SEX
      ),
    .header = "{strata}"
  ) 

If we want to change it at a later stage we still need to use either

1. modify_spanning_header() - could not make it work so far as {strata} levels are not avail maybe? - or

2. we remove them manually in the final flextable step. This would NOT allow to modify them if someone wants to though (I would just grep and remove **).

+   modify_spanning_header(
+     label = "",
+     all_stat_cols() ~ "{strata}"
+   )
Error in `modify_spanning_header()`:
! There was an error in the `glue::glue()` evaluation of "{strata}" for column "stat_1_1".
ℹ Run `gtsummary::show_header_names()` for information on values available for glue interpretation.

Option 1 would be also an issue because we would need to add it everytime or make a tbl_roche_strata wrapper.

Question: should we add strata to show_header_names()?

[Melkiades]

@edelarua @ayogasekaram what do you think?