Sounds like another reason for me to start learning css all over again.
This is 100% TRUE. I've been scrolling and scrolling past hundreds of themes and icons across different websites for xcfe and gnome I could find but none of them really hits the perfect spot for me. Some of them get real close, but theyre not exactly what I want. And boy how much I mind even the tiniest of detail. So that's why modification is important to me.
Can I change these icons to pngs that I want but have the same file name?
Yes, you can. It is just the referenced image, so you really can use any image that you want to. Keep in mind that applications will call on the icon specifying a size. So you want to aim for the following if customizing an icon:
that it is still easily recognizable even at 16x16 px size. Details in the image become very important at small icon sizes.
its colors will be self-supporting allowing them to be used on any background. Avoiding transparent elements is best unless an icon theme is set to be used with a specific system theme.
Simplicity helps. Borders built into the image can help a lot in keeping the icon unique and isolated from its surroundings.
You can copy an icon directory to your home ~/.icons folder, then reassign it user ownership and permissions. If you use the Zorin icon set as a base, you can rename it "zorin-new" or something to make it distinguishable. You can rename it again later if you decide to make a full set.
Once under your ownership and placed in your .icons directory, your system will defer to it as the default icon theme first.
And you will be able to make all the changes you want without going through Root.
You can begin by changing the icons you want and if you should allow yourself to get carried away and change the rest - that is ok, too.
I personally have this preference for non-differentiating the Symbolic icons from the regular - I set the symbolic icons to be exact duplicates of the regular - they look alike.
The .svg files will require Inkscape to manipulate.
sudo apt install inkscape
There is a learning curve to Inkscpae. Image elements in scalar vector graphics tend to be modular portions rather than overlaid layers like they are with rastar graphics. You can select a modular element by left clicking it. You can move it with click and drag. Or remove it with click to select, then ctl+x.
You can also remove all the elements and then size and copy-paste an originally raster image into the field to create it as an .svg image, saving a bit of headache to start out with.
In that past I have fudged my taskbar to be darker than white (white theme) using a degree of panel transparency to let the darker background scene to blend in. I also somehow managed to get white text instead of dark text.
Not ideal for sure, but I could never achieve dark taskbar on light theme in Z15, Z16 Core (gnome). If this is now better served with Z17 Core tools then that would be good to hear.
The taskbar icons don't change instantly when you choose a different icon set. Make sure you logout and login back (or simply do a restart) to see the full changes in panel.