How can I add headers and indent checkboxes when using checkboxGroupInput in Shiny?

I want to add headers to my checkboxes and indent the checkboxes, so they look like this:

Uppercase
  [] A
  [] B
  [] C
Lowercase
  [] a
  [] b
  [] c

Here is my code

letters <- c("A","B","C","a","b","c")
library("shiny")
server <-function(input, output) {
    output$value <- renderPrint({ input$checkGroup })
}

ui <-fluidPage(
         checkboxGroupInput("checkGroup", 
                            label = h3("my letters"), 
                            choices = letters,
         fluidRow(column(3, verbatimTextOutput("value")))
)

runApp(list(ui = ui, server = server))