It’s in the JAR file. But I recommend to not copy it but instead use an additional mapCSS file with the modifications you want. Otherwise it gets hard to apply updates.
Oh thank you very much, I am not versed in Java at all, but I think I can open the jar file and get the file out.
If I understand you correctly, I should get the file out, make a copy, rename it to something of my own and then put it someplace for user based files, leaving the .jar file totally alone?
Where would I put my own .mapCSS file so I could use it in JOSM?
I’m suggesting to create a totally new mapCSS file and having both active. So the new file contains only the things you want to be different. If you want to change a specific object and you don’t know how you would copy only these few lines into your file. So if there’s an enhancement to the original style you’ll still profit from it simply by updating JOSM.