Demo on 'source' tags$style from global.R, alternative to includeCSS("www/style.css")

global.R

# global.R

# customize display settings here
gTags <- function() {
  tags$head(
    tags$style(
      type = 'text/css'
      , "body {font-size: 80%; color: rgb(0,0,100);}"
      , "#mainPanelId {min-width: 50%; max-width: 70%; float: right;}"
      , "#sidebarPanelId {min-width: 10%; max-width: 20%; float: left;}"
      , "h1 {font-size: 20px; color: orange;}"
    )
  )
}

server.R

# server.R

library("shiny")

shinyServer(
  function(input, output, session) {
    
    # Display styles    
    output$main <- renderText({
      paste0(
         "        gTags <- function() {
           tags$head(
             tags$style(
               type = 'text/css'
               , 'body {font-size: 80%; color: rgb(0,0,100);}'
               , '#mainPanelId {min-width: 50%; max-width: 70%; float: right;}'
               , '#sidebarPanelId {min-width: 10%; max-width: 20%; float: left;}'
               , 'h1 {font-size: 20px; color: orange;}'
             ) 
           )
         }"
      )
    })

  }
)

ui.R

# ui.R

library("shiny")

shinyUI(
  pageWithSidebar(
    # Panel 1
    headerPanel("Passing Tags - Demo")
    ,
    # Panel 2
    sidebarPanel = wellPanel(
      id = "sidebarPanelId"
      ,
      h4("Not Much Here")
      ,
      helpText("This is a very minimal demo designed to illustrate how styles can be stored in 'global.R' 
      and how they may be passed to 'ui.R'. 
      Other than that it appears to work, I do not know if it's a commendable approach. 
      As for the layout of the panels, it's lousy but, again, just an illustration.
      Oh and the code you see in the mainPanel, it's not even sourced, it's copy-pasted from 'global.R'.")
    )
    ,
    # Panel 3
    mainPanel = wellPanel(
      id = "mainPanelId"
      ,
      # pass customized display settings stored in function gTags() in global.R
      gTags()
      ,
      tableOutput(outputId = "table")
      ,
      h4("Store your css styles in 'global.R' :")
      ,
      verbatimTextOutput("main")
    )
  )
)