Just installed a new icon theme, from here:
But am missing a few icons: Tutanota and FreeTube. I'm sure the fix is simple but customizing in linux is still very new to me. Can anyone lend a me a hand?
edit: should add that I did check the index.theme file
Name=Besgnulinux-pink-dark
Comment=Multi-colored icon theme
Inherits=breeze,adwaita,hicolor
Is there something here I need to add?
Which Icon theme were you using prior that included the Tutanota and Freetube icons?
You can use that as a reference to know what naming convention to use - then create or find icons to match your existing theme and name them by that convention. Include copies in your icon theme apps folders.
Yes, I thought I might have to go this route. Was hoping there was someway to just point to the missing icons via the index.theme file.
So, do I put a copy of the missing icon in each of these folders of my new theme?
And does image size matter?
Or is it easier to just find a better/complete theme set?
It is better to resize the image to the size designated by the folder.
This is entirely up to you.
I started out the same way- just trying to fix an icon set.
Now, I make my own full sets.
Freetube and Tutanota are not common icons... No matter what icon set you use, there will always be some missing.
So it helps to get used to adding your own from time to time.
It becomes very easy once you know what you are doing.
I know. I've seen your other posts. Impressive.
Exactly. Do it to learn it. There are a number of things I want to change and/or fix but 90% of it I can't find the solution online or in Zorin forums. It's frustrating.
Hope you don't get sick of me over the next week as I work on these.
Probably not.
I'd much rather teach to fish than hand them out.
I'll bet those that hang around with you do too .... 
...casts her rod into the still glass lake.
Okay Sensei, this grasshopper is ready to "wax on, wax off." Get ready. 
I think there will be a few more of us that will learn right along with you .....
Exactly! 
What are you using to save the image as an svg?
If you are saving a file that is in a raster format, like .png and you want to save it as an .svg, you must convert the file.
You can use a simple terminal command to do this on Zorin OS since Zorin OS comes with imagemagik installed. Run the terminal command from within the same directory as the the files:
convert example.png example.svg
I use Inkscape as a graphical editor for .svg files. Inkscape is a versatile and powerful app.
Inkscape includes cli commands as well and the list is extensive.
But... If you install Inkscape, you can convert using the GUI save function.
Open a .png in Inkscape, then save it as an .svg
You can also open .svg file in Inkscape, then export it as a .png image File > export > png image or shift+ctrl+e
Yes, thank you. Already knew this. The design aspect of customizing is the easy part. Finding the files and where everything hides is the challenge, but I'm getting there.
As you said earlier... learn as you go.
I use Inkscape. 
This is the solution, but for whatever reason I am not getting an option to mark this as the solution. Only options are to like, share,, flag or bookmark... weird.
@Storm taught me a much better and more organized method - and one that saves space (bloat) and is simpler to work with.
To use as a template, you can download one of Storms icon sets (I Like Mistique set)
Or install one of mine
sudo add-apt-repository ppa:aravisian/iconsets
sudo apt-get update && sudo apt install antares-icons
It does not matter what the Icons look like really - since you would be using the set as a template to replace the icons with the ones that you prefer.
The Short Version is: The set uses an index to determine icon sizes, with .svg files for regular and symbolic icons. Much more concise than having differently sized directories in a series...
The thread is in Customization which is a discussion subforum, not a HelpDesk subforum. So these threads often do not have One Notable solution.
This is invaluable. Wow. Thanks so much for this. Will make life so much easier in the future.
Ooooh, doh. Good to know. Thx!
Just letting you know that after updating the repository, when I entered:
sudo apt-get update && sudo apt install antares-icons
My system updated, but the icons didn't install. Had to run the below again separately in order for it to work.
sudo apt install antares-icons
The && means, perform the second action if the first action was successful.
Something (packet drop?) may have interrupted the sudo apt update command causing it to not fully complete.
Yes, actually the update had a 404 error for conky. Didn't know it prevented actions following it.
Err:25 http://ppa.launchpad.net/mark-pcnetspec/conky-manager-pm9/ubuntu focal Release
404 Not Found [IP: 185.125.190.52 80]
and
E: The repository 'http://ppa.launchpad.net/mark-pcnetspec/conky-manager-pm9/ubuntu focal Release' does not have a Release file.`
I'll have to figure out what to do about that. Thanks for clarifying.