How to remove italic font style from the editor font?
look for places where it's italic in Colors And Fonts, then disable it.
Editing the color theme settings files directly seems to work:
sed -i '/<option name="FONT_TYPE".*/d' <jetbrains settings dir>/colors/*
e.g.
sed -i '/<option name="FONT_TYPE".*/d' ~/Library/Application Support/JetBrains/WebStorm2020.1/colors/*
Most helpful comment
look for places where it's italic in Colors And Fonts, then disable it.