In RoboHelp 2019, I used WebHelp to CHM to generate a CHM file that had a skin. In the RH2020, there is no WebHelp to CHM option that I can find. If I generate a Microsoft HTML Help output, the CHM is generated without a skin. I can't figure out a way to get the skin on this output.
The converter was only in 2019 Classic. 2019 New UI and 2020 both have very different code and the converter could not be ported. Another factor is that strictly speaking 2020 does not have webhelp in the same sense as Classic versions. You either use the frameless (recommended going forward) or responsive skins. They can be set to not resize as the window size changes so that they are effectively the same as webhelp.
Sorry but there is no workaround.
The About RoboHelp sample project that ships with 2020 covers the new features and the changes.