Can you create a sample project and share with dropbox, google drive, …?
It will be easier for someone to just run that and see if something can be changed to make it work. If not, it can be used to create an Issue either for bug report or feature request.
Edit: I guess you want the ‘grid’ to be same color as the text labels? This is a screenshot from the Web Example that comes with Xojo:
Edit2: I’m having a hard time finding if ChartJS offers automatic dark mode. Maybe this will be a feature request to Xojo offer options that are not available automatic on the underlying code.