How do you format multiple columns under the same spanner? / How does gt() assign column location?

I have a gt table that has a series of spanners. Each spanner has two columns beneath it. I'd like to format it such that the columns under every other spanner are highlighted. Columns 1 has the stub values. In the code below, column 2 is the location of the first column under Spanner 1. I would expect column 3 to highlight the column directly next to 2, but instead it skips that column and highlights the first column of the second spanner (what I would call column 4).

my_data %>%
 gt() %>%
 tab_spanner_delim(delim = "_") %>%    
 tab_style(
       style = list(cell_fill(color = "#f2dddc")),
       locations = cells_body(columns = c(2,3)) # 6,7,10,11, etc. for every other spanner
           )

So how do I format my code to reference both columns under a spanner/how do I reference any column under a spanner that isn't the first column? Is there a way to reference the spanner as the location reference?

And for what it's worth, I'm doing this within Shiny, so once I understand the mechanics of gt() I will need to further modify the code to make it dynamic.

Thank you!

Update: I'm having similar issues with referencing spanners. The code below successfully adds a left border starting from the spanner and running to the bottom of the column on columns 2 and 3 (so the first spanner and its 2 columns are wrapped in outer borders).

%>%
        tab_style(
          style = list(cell_borders(
            sides = "left",
            color = "black",
            weight = px(3)
          )),
          locations = list(cells_column_labels(columns = c(2,3)), #left side of column two and 3 (remember col 3 is the first column of spanner 2)
                           cells_body(columns = c(2,3)), #Repeat of above but with the column values
                           cells_column_spanners(spanners = c(1,2)) #Left sides of spanners 1 and 2 (left side of spanner 2 = left side of col 3)
        )) #Results are as expected

Now, I want to highlight the name of the first spanner. Given the code chunk above, it makes sense to me to reference spanner 1. When I reference spanner 1, the second spanner is highlighted. Any idea why? I don't understand why I can get a border to the left of spanner 1 by referencing '1', but when I make the same spanner reference to highlight the cell, it highlights spanner 2. I then tried referencing spanner 0 and it threw an error that the indice didn't exist.

%>%
     tab_style(
      style = list(
         cell_fill(color = "#f2dddc")),
       locations = list(cells_column_spanners(
         spanners = 1)) #Why does this line highlight spanner 2?
              )

Solution

I've come to realize that the (my) confusion lies in the tab spanner delimiter. I used pivot_wider to prepare my data for gt() resulting in a df formatted as such: (1) Group1_varA, (2) Group2_varA, (3) Group3_varA, (4) Group1_varB, (5) Group2_varB, (6) Group2_varB.

The tab_spanner_delimiter rearranges the columns so varA and VarB are under their respective groups, but the column indices don't change, making the column orders in the above example 1,4,2,5,3,6.