I’ve never written a Manifest v2 extension, so I can’t say.
These two threads seem to have similar code working in v2, though:
- How to correctly design/color icon to be visible across themes?
- Add-on SVG toolbar icon: How to fit the color scheme of the current theme
I haven’t been able to find a v3 extension with dark/light icon variants, yet—have you run into any working sample?
Of note, I can actually reproduce the issue in Chrome (version 109), suggesting there is something wrong with my manifest.
(and in Safari, for which the extension was originally developed, the image is in my case treated as a mask anyway, so the light/dark variants don’t matter)