Classic designer: highlighted selected button in Choice control similar to Modern Designer

You can try using a Formatting form rule on the Choice control. It might not be exactly what you are looking for, but maybe close enough. Below are the steps to configure:

• Select the Choice control and add a rule to it.
  - Name = Option Formatting (or whatever you would like to name the rule)
  - Rule Type = Formatting
  - Condition = not(isNullOrEmpty(Option Select))
  - Formatting - Choose a desired fill color, text color, font, bold, italics, etc.
• Change the sizing of the Choice control so that there is no space below and to the right.
• The images below are the configuration and the results on the form.

Close!  Unfortunately, my list of choices spans several rows, so no rectangle will encompass it.  I placed it in a panel and made the panel background match the selected color, and that seems to look better.