You are welcome to contribute icons. Many of the icons in calibre are simply collected from different places. It would be nice to have a consistently styled set of icons.
However, changing icons has a cost, it upsets users who have become used to a particular set of icons. So, there are two possible approaches you can take:
1) You can work on icons that are somewhat less common first, changing them to be consistent with the style of the main icons
2) You can create a full custom icon set and release it like the other icons sets here:
https://www.mobileread.com/forums/sho...d.php?t=151397
If you do (1) and work a few icons at a time, with continuous feedback from me, your work is likely to be accepted into main calibre quickly.
If you want to do (2) then you will need to agree to a set of guidelines for the new icon set and an overall look/scheme for them. See for example my comments here:
https://github.com/kovidgoyal/calibre/pull/175