首先，这里是一个改变CSS内联的方法：

library(shiny)
library(plotly)

shinyApp(ui = navbarPage(title = "Thai Economic",
                         
      tags$head(
        tags$style(HTML("
      body {
        background-color: black;
        color: white;
      }"))
                         ),
                         
                 # Global Economic Development Page.
                 tabsetPanel(
                   tabPanel("Population",
                          id = "population",
                          h1("Thailand Population"),
                          br(),
                          plotlyOutput("thai_population_chart"),
                          br(),
                          fluidRow(
                            column(width = 2, actionButton("pop_line","Line Chart")),
                            column(width = 2, actionButton("button_id", "Bar Chart"))
                          ),
                          br(),
                          p("Over the last decade, Thailand's population has continued to grow steadily. 
                According to the World Bank, the population of Thailand in 2011 was approximately 68.3 million, 
                while the estimated population in 2020 was approximately 69.8 million. 
                This represents an increase of around 1.5 million people over the course of the decade. 
                Despite this growth, the rate of population increase in Thailand has been slowing down in recent years, 
                due in part to a declining birth rate and an aging population. Nonetheless, 
                Thailand remains one of the most populous countries in Southeast Asia, with a diverse population that includes a mix of ethnic groups, 
                religions, and languages.")
                
                 ),
                
                
                
                # Thailand GDP Page
                tabPanel("GDP",
                         h1("Thailand GDP"),
                         br(),
                         plotlyOutput("thai_gdp_chart"),
                         br(),
                         fluidRow(
                           column(width = 2, actionButton("gdp_line", "Line Chart")),
                           column(width = 2, actionButton("gdp_bar", "Bar Chart"))
                         ),
                         br(),
                         p("Thailand's...")
                ),
                
                # Poverty Page
                tabPanel("Poverty",
                         h1("Thailand Poverty"),
                         br(),
                         plotlyOutput("thai_poverty_chart"),
                         br(),
                         fluidRow(
                           column(width = 2, actionButton("pov_line", "Line Chart")),
                           column(width = 2, actionButton("poverty_bar", "Bar Chart"))
                         ),
                         br(),
                         p("Over the past decade....")
                ),
                
                # Import and Export goods and Service Page
                tabPanel("Import and Export goods and Service",
                         h1("Import and Export goods and Service"),
                         br(),
                         fluidRow(
                           column(width = 6, plotlyOutput("thai_imports_chart")),
                           column(width = 6, plotlyOutput("thai_export_chart"))
                         ),
                         br(),
                         fluidRow(
                           column(width = 3,
                                  actionButton("im_line", "Line Chart"),
                                  actionButton("im_bar", "Bar Chart")
                           ),
                           column(width = 3, offset = 6,
                                  actionButton("ex_line", "Line Chart"),
                                  actionButton("ex_bar", "Bar Chart")
                           ),
                         ),
                         br(),
                         p("Thailand is a major ...")
                ))
                
),
server = function(input, output, session) {
  
})

或者，将head标记替换为以下代码，并将custom.css放在www文件夹中：

tags$head(
    tags$link(rel = "stylesheet", type = "text/css", href = "custom.css")
  )