{dRacula} provides a nice looking UI theme for Shiny.
You can install the development version of {dRacula} like so:
remotes::install_github("RinteRface/dRacula")This is a basic example which shows you how to solve a common problem:
library(dRacula)
shiny::shinyAppDir(system.file("examples/base", package = "dRacula"))