" It's not as if someone would ask for an OS/2 or a Motif UI theme! There are *LOTS* of people on Windows still !" That is irrelevant really . Thats not how open source works. In open source something maintained or goes in or stays in because someone bothers to code for it.
You may get 1 billion users to complain about this and still it wont change a thing. You still need a developer to do the work. Most open source devs like to work on things that they really care about. And is not as if things are better for MacOS user, the watery theme looks very dated , ugly and it is not even compatible with the new dark theme. So in the end it makes sense to use one of the recent themes, either white or dark. Also you are the first to complain about this.If you want to get this fixed the easiest way is to fix it yourself as third party library and add it to the configuration browser for others to use. If you expect for someone else to fix it, you may have to wait for decades and no it wont matter if you get a billion users to agree with you and sign a petition. How many people you think asked for a blue theme in this mailing list ? none . Pharo now has a blue theme because of me, and has no windows theme with so many windows users. How many people use the blue them ? most likely only me. Thats how open source works I am afraid, dont blame Pharo, blame life and the reality we live in :)
