Customize icons: some icons I can't find

I want to customize the save icon that is used when the document requires saving.
With whatever theme I’m using the icon is a green down arrow. When the document requires saving (has been edited) there is a small red dot in the upper right corner.

It’s that icon I wish to change. I’ve changed the save icon, that was easy.

This feature uses two icons really: one for the unsaved state, and one for the saved state. And you can modify only one of them in the Customize feature.
I do not know where the actually selected icons are stored.

AFAICT
Each icon theme contains files (.png or .svg) named like: savemodified_extralarge, savemodified_large and savemodified_small (cf. source code).

Test done on Windows 11 (installled LO 24.8.3.2) for the Colibre theme.

Themes as .zip archives are in the LibreOffice folder in (...)\share\config
The relevant icons reside in the archive’s subfolder \res

One has to replace the said images in the archive(s) to see the new icons in the UI.

My apologies for not uploading the appropriate recording — voilà!

recWin11_24832

I was able to get it working for colibre. I tried Yaru blue svg file but didn’t seem to get it working.
It works now as I have it, and I fully understand the process.
Thanks for all the input.

The solution I used is to download the extension at

Change the .oxt to .zip and extract it.
Within this find the images_yaru_blue.zip and extract it.
Find the save and savemodified icons and change them.
Move into the images_yaru_blue_svg folder and select all files and compress to a zip. I saved it one layer up as: …/

Replace the original images_yaru_blue_svg.zip file that came with the oxt file.
Move into the yaru-blue-theme folder and select all files and compress to a zip, also to the folder one layer up. …/

Change the zip extension to oxt.
Install in Libreoffice.
Save this somewhere for when LO has to be reloaded.