devtools.panels.create [1] receives the path to icon, that is shown in the panel's tab,
the icon can be specified only when creating the panel, and cannot be changed later.

Then, you can switch the DevTools' theme in Settings panel, and the change is immediately
reflected to the UI, leaving extensions' panel icons as is.

devtools.panels.onThemeChanged [2] provides a way to detect the theme switch, but with lack of the way to change the icon on already-created panel, the there's no way to reflect the theme change to the icon.

this is problematic if the extensions follows the current icon style (monotone icon),
given light-theme icon on dark-theme (or vice versa) is almost invisible.

[1] https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/devtools.panels/create
[2] https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/devtools.panels/onThemeChanged

Bug 1617554 is related, that the way to reflect the theme change to icon that built-in panels are using doesn't work for extension's panel.

So, currently, the only way to reflect the theme switch to extension's icon is to close the DevTools and reopen it.

Clearing needinfo, I replied to the related needinfo on Bug 1618580 (Bug 1618580 Comment 2).

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

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