I find the old stylesheet UI clunky and hard to use:

    FrontEndExecute @ FrontEndToken @ "EditStyleDefinitions"

[![enter image description here][1]][1]

Without using the above, I'd like to programmatically quickly tweak and preview the style definitions for named styles. 

For example, let's try copying `"Entity"` and renaming it `"MyRoundedFrameStyle"` giving it different colors and frame properties:

    CellPrint @ Cell[BoxData[
        TemplateBox[{"\"United States\"", , , "country"}, "Entity"]], 
      "Input"]

[![enter image description here][2]][2]

**Notes:**

  - Being able to do this inline and programmatically within a notebook would "be best" ;)
  - I haven't seen any [ResourceFunction][3] that could help here...
  - Perhaps there are some undocumented symbols that let one ergonomically copy-and-tweak (predefined cell or box) styles for quick reuse?
  - If nothing like this exists, `StylePlayground` would be a good name!



  [1]: https://i.sstatic.net/Jgx11.png
  [2]: https://i.sstatic.net/lcHA5.png
  [3]: https://resources.wolframcloud.com/FunctionRepository/search/?i=style