最简单（也是最快）的方法是使用一点JavaScript

library(shiny)

ui <- navbarPage(
  id = "navbar",
  tags$style(HTML("
    .tabbable > .nav > li > a {color:blue}
    .tabbable > .nav > li.active > a {color:red}
    .tabbable > .nav > li > a.red_panel {color:red}
  ")),
  
  tabsetPanel(id="tabs", 
              tabPanel(value = "tab1", title= "Tab1"),
              tabPanel(value = "tab2", title= "Tab2"),
              tabPanel(value = "tab3", title= "Tab3")
  ),
  
  # We could also append the style directly using .css()
  tags$script(HTML("
    $('#tabs').on('shiny:inputchanged', function(event) {
      console.log(event.value)
      if (event.value === 'tab2') {
        $('a[data-value = \"tab3\"]').addClass('red_panel');
      } else {
        $('a[data-value = \"tab3\"]').removeClass('red_panel');
      };
    });
  "))
)

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

shinyApp(ui, server)

我不确定我是否理解你的问题，但如果我理解的话，这就是你想要的：

css1 <- ".tabbable > .nav > li > a {color: blue;}
.tabbable > .nav > li[class=active] > a {color: red;}
"

css2 <- ".tabbable > .nav > li > a {color: blue;}
.tabbable > .nav > li[class=active] > a {color: red;}
.tabbable > .nav > li > a[data-value=tab3] {color: red;}
"

# ui
ui <- navbarPage(
  id = "navbar",
  uiOutput("CSS"),
  
  tabsetPanel(id="tabs", 
              tabPanel(value = "tab1", title= "Tab1"),
              tabPanel(value = "tab2", title= "Tab2"),
              tabPanel(value = "tab3", title= "Tab3")
  )
)

# server
server <- function(input, output, session) {
  
  output[["CSS"]] <- renderUI({
    css <- ifelse(input[["tabs"]] == "tab2", css2, css1)
    tags$style(HTML(css))
  })
  
}

shinyApp(ui, server)