title: How to modify the themes of shinythemes?
body: <p>How to modify the themes of <code>shinythemes</code>? For example, let us assume I want to change <code>darkly</code>'s background black.  </p>
  
  <pre><code>library(shiny)
  library(shinythemes)
  
  ui &lt;- fluidPage(
    theme = shinytheme("darkly"),
    "How to change this black?")
  
  server &lt;- function(input, output) {}
  
  shinyApp(ui, server)
  </code></pre>
  
  <p>I guess one solution would be to copy the CSS of <code>darkly</code> with the modified black to the www-folder of my app and then use <code>theme = "darkly_modified_black.css"</code>. Is there a simpler solution I am missing?</p>