It seems that Claws applies "fg" as a color of text, not "text". See attached image. I've changed "fg" color for Tree* widgets to red to better highlight the issue and in theme preview it displays correctly, fg used for column titles, "text" for text (I've used gtk-theme-switch2 as an example). Now in Claws as you can see it uses fg instead of text which makes it unreadable if theme have light-on-dark column headers but dark-on-light content
Created attachment 1795 [details] screenshot of bug
Created attachment 1842 [details] A fix Attached patch should solve the issue. I tried with fg (all states) set to red and text (all states) set to green, and everything seems to still work (custom folder colors in folderview, message color flags in summaryview, etc.).
Changes related to this bug have been committed. Please check latest Git and update the bug accordingly. You can also get the patch from: http://git.claws-mail.org/ ++ ChangeLog 2018-02-14 22:59:04.712209603 +0100 http://git.claws-mail.org/?p=claws.git;a=commitdiff;h=dde11a75c75f469eb3bccf139566841844c154f4 Merge: d0cc31b 13d82d6 Author: Colin Leroy <colin@colino.net> Date: Wed Feb 14 22:59:03 2018 +0100 Merge branch 'master' of file:///home/git/claws http://git.claws-mail.org/?p=claws.git;a=commitdiff;h=13d82d666b2ef07f1ae58ba5c44c889205ced720 Author: Andrej Kacian <ticho@claws-mail.org> Date: Tue Feb 13 23:50:15 2018 +0100 Make GtkCMCList/Tree use "text", not "fg" rc property for text color. Closes bug #3880.
Fixed in git. Thanks for bringing this up!