Comments in bug report 1188945 say to use Error Console, enter
"Application.platformIsLinux" and click Evaluate. I get no response at all
from that. But I get "false" back from evaluating "Application.platformIsMac".
Based on those bug report comments, I changed 2 Lightning .js files in my
extensions directory to remove the tests for Application.platformIsLinux
and assume that was true. This fixed the problem for both calendar and
category color dialogs.