Claws task icon is smaller than others in Windows 8 camouflage mode

The claws icon is smaller than other icons, as reported by intrigeri on tails-dev. I've verified that icons/apps/claws-mail.png@ in gnome-theme-windows8 have the correct size, so I guess it’s claws fault (?).

Parent Task: #7377 (closed)

Original created by @anonym on 7397 (Redmine)