When you need to add a specific style to an existing element generated by a package, there are various approaches you can take:

1. One method is to wrap the element itself in a div with the desired style, although this may not work for all CSS elements.

2. You can create a custom function using the source code of the element you're interested in. For example, you can refer to the source code from this link.

3. Another approach is to incorporate external CSS that specifically targets the element you want to modify. However, this method is less preferred as it separates the styling logic from its application location, requiring you to manage it for each element individually.

library(shiny)
library(shinyWidgets)

# Custom function for the second approach
actionBttn_with_style <- function(inputId, label = NULL, icon = NULL, style = "unite",
                       color = "default", size = "md", block = FALSE,
                       no_outline = TRUE, my_additional_style = "") {
  value <- shiny::restoreInput(id = inputId, default = NULL)
  style <- match.arg(
    arg = style,
    choices = c("simple", "bordered", "minimal", "stretch", "jelly",
                "gradient", "fill", "material-circle", "material-flat",
                "pill", "float", "unite")
  )
  color <- match.arg(
    arg = color,
    choices = c("default", "primary", "warning", "danger", "success", "royal")
  )
  size <- match.arg(arg = size, choices = c("xs", "sm", "md", "lg"))

  tagBttn <- htmltools::tags$button(
    id = inputId, type = "button", class = "action-button bttn", `data-val` = value,
    class = paste0("bttn-", style), 
    class = paste0("bttn-", size),
    class = paste0("bttn-", color), list(icon, label),
    class = if (block) "bttn-block",
    class = if (no_outline) "bttn-no-outline",
    style = my_additional_style
  )
  shinyWidgets:::attachShinyWidgetsDep(tagBttn, "bttn")
}

Once you have created your customized button function, you can use it similar to actionBttn within your ui.

ui <- dashboardPage(
  dashboardHeader(
    title = "example"
    ),
  dashboardSidebar(),
  dashboardBody(

    # Adding external CSS for approach #3, although this separates the styling logic
    htmltools::tags$head(
      htmltools::tags$style('button#clear_ext_css { margin-top:25px }')
    ),

    box(
      width = 12,

      column(
        width = 2,
        dateRangeInput(
          "dateRange",
          "Date Range",
          start  = "2017-01-01",
          end    =  Sys.Date(),
          min    = "2001-01-01",
          max    = Sys.Date(),
          format = "mm/dd/yy",
          separator = " - "
        )
      ),

      column(
        width = 1,
        actionButton('Apply', 'Apply', style = 'margin-top:25px')
      ),

      column(
        width = 3,
        # Applying approach #1 by wrapping it in a styled div
        div(
          actionBttn(
            inputId = 'clear_div',
            label = "Clear with div",
            style = "gradient",
            color = "danger"
          ),
          style = 'margin-top:25px'
        )
      ),
      column(
        width = 3,
        # Using the custom function from approach #2
        actionBttn_with_style(
          inputId = 'clear_fn',
          label = "Clear with custom function",
          style = "gradient",
          color = "danger",
          my_additional_style = 'margin-top:25px'
        )
      ),
      column(
        width = 3,
        # Implementing approach #3 with custom CSS
        actionBttn(
          inputId = 'clear_ext_css',
          label = "Clear with external CSS",
          style = "gradient",
          color = "danger"
        )
      )


    )
  )
)

server <- function(input, output, session) {

}

shinyApp(ui, server)

https://i.sstatic.net/1Ekof.png

Responding after some time in case this information is still relevant.

To modify the actionbttn element, you can make the necessary changes in a .css file located in the www directory.

The selector for your button should be as follows:

button#submit.action-button.bttn.bttn-bordered.bttn-md.bttn-default.bttn-no-outline.shiny-bound-input 

Your .css file should be structured like this:

button#clear.action-button.bttn.bttn-gradient.bttn-md.bttn-danger.bttn-no-outline.shiny-bound-input
{
  margin-top:25px
}

I chose 25px as it provided better alignment:

https://i.sstatic.net/BBeky.png

Below is the code snippet:

library(shiny)
library(shinydashboard)
library(shinyWidgets)

ui <- dashboardPage(dashboardHeader(title = "example"),
                    dashboardSidebar(),
                    dashboardBody(tags$head(
                      includeCSS('www/style.css'),
                      box(
                        width = 12,
                        
                        column(
                          width = 3,
                          dateRangeInput(
                            "dateRange",
                            "Date Range",
                            start  = "2017-01-01",
                            end    =  Sys.Date(),
                            min    = "2001-01-01",
                            max    = Sys.Date(),
                            format = "mm/dd/yy",
                            separator = " - "
                          )
                        ),
                        
                        column(
                          width = 3,
                          selectizeInput(
                            inputId = 'var',
                            label = 'Select variable',
                            choices = c('cut', 'color'),
                            multiple = FALSE,
                            options = list(
                              maxItems = 1,
                              placeholder = '',
                              onInitialize = I("function() { this.setValue(''); }")
                            )
                          )
                        ),
                        
                        column(
                          width = 1,
                          offset = 2,
                          actionButton('Apply', 'Apply', style = 'margin-top:25px')
                        ),
                        
                        column(
                          width = 3,
                          actionBttn(
                            inputId = 'clear',
                            label = "Clear",
                            style = "gradient",
                            color = "danger"
                          )
                        )
                      )
                      
                    )))

server <- function(input, output, session) {
  
}

shinyApp(ui, server)