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... ...
