RoboHelp v2017 doesn't recognize my CSS when using CSS variables

I have a RoboHelp v2017 question related to CSS variables.

We're trying to implement CSS variables in our style sheet but when I implement it in RoboHelp, the Design view and the Preview Topic view (Ctrl+W) doesn't recognize the style sheet at all. Once I generate or publish it in Responsive HTML5 output it looks great though.

Does anyone know if this is a reported bug? If so, is there a targeted version of RoboHelp that fixes this?

It's especially annoying for large projects to have to generate the project each time a new variable is introduced to test/review the update when I should be able to just do a preview of a topic from within the RoboHelp application. It also seems odd that Adobe would release an HTML5 output without taking all possible CSS3 enhancements into consideration.

Thanks!