This is GTK. Which (still) can be themed, though that is harder today, than in the times of GTK2. But maybe these https://www.claws-mail.org/themes.php suffice? Those change just some icons. I used it in the past, and had no problems with it(after theming). It was much more light than thunderbird, at the times, so it could run in the background without dragging other(desktop) stuff down.