When customizing one's desktop, there are always few applications that resist the change. This time I was fighting OBS Studio under Ubuntu and its dreadful tray icon. It just didn't fit with my desktop theme.
I searched around and found essentially nothing. Yes, there were pages upon pages of people having the same problem and then other people giving them generalized advice but without anything actionable.
To make long story short, it took understanding Qt theming to find a solution for me. It's not an ideal solution since it does require modification to the theme that could be lost in the future update but the trick was in getting icons added to the /usr/share/icons/hicolor/ directory.
I simply took my chosen 16x16 icons and placed them at the following locations:
/usr/share/icons/hicolor/16x16/status/obs-tray.png/usr/share/icons/hicolor/16x16/status/obs-tray-active.png/usr/share/icons/hicolor/16x16/status/obs-tray-paused.png
For a good measure I also changed their 24x24 and 32x32 variants and that was it. OBS now picked up my icons without an error.
PS: And no, adding them to .icons will not work since it'll break the rest of the system's theme. And yes, one could go around that by copying the whole theme to the directory but I found it easier just to copy a few files to the system directory instead.