How to change gtsummary theme's footnote marks as subscript instead of superscript

Question

Under "Writing Themes" in gtsummary vignette there is an option to change default expression of as_gt and as_flex_table

I need to change table's footnote marks as subscript instead of superscript in my new custom theme

Following the stackoverflow example, I can do the following:

glm(response ~ age + stage, trial, family = binomial) %>% 
  tbl_regression( exponentiate = TRUE) %>% 
   modify_table_styling(
    columns = everything(),
    rows = !is.na(variable),
    missing_symbol = "NA"
  ) %>% as_gt() %>%
  gt::opt_footnote_marks(marks = "letters") 

However, I don't know how to implement this in a theme for both as_gt and as_flex_table. I tried the following for as_gt:

my_theme <-
  list(
 "as_gt-lst:addl_cmds" = list(
          tab_spanner = list(
            rlang::expr(gt::opt_footnote_marks(marks = "letters"))
          )

I don't see any changes ?!