I want to create my own custom theme in power bi, I need to set custom font for secondary axis in line chart. (power bi adds secondary axis in line chart in new release May 2020 )how to write json code?
The report theme JSON format is outlined in the MS Docs web site, with the structure and contents required. You can also export a theme from Power BI, update it and add it back to the PBI File.
What will be easier to do is to use a theme builder, the best one I've go across is at the PowerBI Tips site or the older version, that allows to to select colours and add attributes to the visual.