[Users] Issue - background color in current email message line (Subject; ...)

[Victoria Stuart (VictoriasJourney.com)]

Arch Linux:

Claws Mail: default internal theme
xfce4: Adwaita theme

Issue: Claws Mail theme is internally coded, independent of system desktop
(xfce4) theme?  The highlighting of selected messages (dark blue background
color - see screenshot here: https://imgur.com/a/VnjEMCs ) is way too dark.

How can I change / edit it?