DevTools panels' icons are using SVG fill="context-fill" fill-opacity="context-fill-opacity" to reflect the Dark/Light theme,
but this doesn't work on extensions' icon.

attached testcase uses the exactly same SVG file as "What's New" panel [1], for the extension's panel's icon.

Steps to reproduce:

1. Run Nightly 75.0a1 (2020-02-23) (64-bit) on macOS 10.14.6
2. Open about:config and set xpinstall.signatures.required to false
3. Install attached extension
4. Open DevTools
5. Open DevTools' Setting
6. Switch between Dark/Light theme

Actual result:

• at step 4, the extension's panel icon is shown with black color (while "What's New" icon is a bit gray, because of fill color and opacity)
• at step 6, the extension's panel icon doesn't reflect the theme color, but keeps black color

[1] https://searchfox.org/mozilla-central/source/browser/themes/shared/icons/whatsnew.svg#6

I'm pretty sure there is an existing bug for this but I can't find it so I'm adding a few related bugs in the meantime.

From what I understand, basically we've wanted to do this for a long time, but we don't want to ship non-standard things to Web extensions if we can avoid it. At the same time there's never been anyone assigned to driving the standards process for this although there have been many proposals in this area over time (SVG parameters, something to do with CSS variables, env() etc.). So for the time being we've added hacks to expose this in limited ways to fix Mozilla's use cases without fixing it for developers at large.

So ultimately, I suspect this is a layout bug.

The priority flag is not set for this bug.
:mixedpuppy, could you have a look please?

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