@kiwidude These are the new icon set

(that give you a clue about the old ones

)
Except for the 'matched set' requirement, you can place your own icons in
Your (private) calibre configuration directory for any or all. just use the exact same name as the programs resources folder has
from time to time: Users have posted alternate Icon packages here at MR (search)
Calibre's functionality is #1
Calibre's cross platform is #1.1
Pretty GUI is way down on my list.
Near the top is... Just:
Make it work.
Make it work better (converting).
Make it work Faster
We have gained, over time:
Customize Menus
Alternate-Custom Edit Metadata forms
customized field value display
Custom Fields, including User chosen composite values
I don't agree with all the placement (there may be unknown to me cross platform restrictions)... But I would
never say it is terrible