Replace body with javascript

example.R

library(shiny)

prepare_party <- function() {
    tags$script(
        HTML('
        function replaceBody() {
         document.body.innerHTML = "<h1 style=\\"font-family: Gothic Sans; font-size: 20pt; color: blue;\\">HAPPY BIRTHDAY</h1>";
        };
      ')
    )
}

party_time <- function() {
    shinyjs::runjs("replaceBody();")
}

ui <- fluidPage(
    tags$head(
        shinyjs::useShinyjs(),
        prepare_party()  
    ),
    actionButton("testButton", "Celebrate"),
    tags$h1("no party =(")
)

server <- function(input, output, session) {
    observeEvent(input$testButton, {
        message("Staring party")
        party_time()
    })
    
    
}

shinyApp(ui, server)

Note you'll need to run the shiny app in the browser to see the effect