Let's give the whole application a lick of paint using the shinythemes package. If you haven't already done so, install it with install.packages("shinythemes"). The documentation (including a list of the available themes) can be found at rstudio.github.io/shinythemes/. Load the package and pass a theme into fluidPage():

library(shinythemes) 
fluidPage(
  theme = shinytheme("darkly"), 
  ... rest of UI... 

If you want to choose your theme interactively, instead add themeSelector() to your UI definition, and a little interactive chooser will appear on your app. Once you're happy with it, use the previous code format to make that the default choice in your app:

library(shinythemes) 
fluidPage(
  themeSelector(), 
  ... rest of UI... 

Enjoy your new theme and consult the documentation for the other available themes and the appearance of each.

And here's the finished application, showing the spinning penguin and the user and calendar icons, as well as the theme chooser and one of the themes: