In case anyone else wants a "quick and dirty fix" for this, here are the Monstre icons correctly named. To install them:
- Get the icons from the zip file
- Put them in the folder "resources/images" contained in your calibre configuration folder (get there via calibre / Preferences / Miscellaneous / Open calibre configuration folder).
Putting an icon in resources/images overrides an icon found in an icon pack or the default calibre theme. If you change from Monstre then you should remove or rename these files.