I use Claws Mail v. 3.17.8 in an XFCE4 desktop environment on Debian Bullseye.

When I apply a GTK+ color theme via XFCE's `Settings → Appearance` dialog, Claws Mail *mostly* applies it as expected. Window backgrounds, buttons, dialog boxes, folder/message list views... all good. Except...

If `Preferences → Display → Colors → Other → Enable coloration of message text` is enabled in Claws Mail, it appears to offer only two options: the Claws Mail default colors (statically defined regardless of GTK+ theme) or manually picking the various text (and optionally background) colors one by one. For example, setting my desktop to the Solarized-Dark GTK+ theme successfully sets the message view pane background to dark aqua and the basic message text to the appropriate light color per the theme, but quoted text still uses Claws' static default, which gives a near-unreadable blue-on-aqua result. Unless I'm missing something, this means that Claws Mail's GTK+ theme support is unfinished.

I'm no GTK expert, but presumably among the theme's many classes and styles, Claws should be able to grab sane theme-respecting defaults for the items under "Enable coloration of message text". I would think that the correct hierarchy would be: colors manually chosen by the user override both the theme and Claws' built-in defaults; the pre-selected default colors in that Preferences dialog should be taken from the GTK+ theme; and Claws' statically-defined defaults should only come into play if some problem leaves Claws unable to retrieve the GTK+ theme colors.