When you are designing an UI, you can use HTML tags function of shiny to write the html part. Use shortcut like h1 or p, or use tags$h1 and other inside thetags element.
If you replace your textOuput part with that, you can write your UI.
library(shiny)
ui = navbarPage("Whatever", inverse = TRUE, collapsible = TRUE,
tabPanel(title = "Home",
fluidRow(
column(5, offset = 0,
navlistPanel(
tabPanel(title = "About",
br(),
h1("About text"),
p("Explain who we are")
),
tabPanel(title = "Contact Us",
br(),
h1('How to contact us?'),
p("Tel & mail")
)
)
)
)
)
)
shinyApp(ui, server = function(input, output){})
For styling, you can play with css then. (i.e for red title).
Read articles about UI and follow the tutorials on the shiny website to be accustom with all this.