The Mark all topics read in Chrome does not work. I have not tried it in other browsers, but here is my workaround in Chrome.
- wrench icon
- Clear browsing data
- In Clear browsing data popup, in the Obliterate the following items from: select the beginning of time, or whatever period you wish.
- Check Clear browsing history
- Uncheck everything else
- Click on Clear Browsing Data box lower right in popup
- Return to Contest Galaxy. Everything should be returned to default blue, except for those deliberately changed to a different colour.