First, build your gt_summary table.
Then convert it to a format, with as_gt() or as_flextable().
Then use the many formatting commands in the gt or flextable packages to do your formatting.
For example, with gt()
library(gt)
library(gtsummary)
trial %>%
tbl_summary() %>%
as_gt() %>%
tab_options(column_labels.font.size = 24,
column_labels.background.color = "cadetblue",
table.background.color = "blanchedalmond")